[MRG] DOC Adds _pairwise property to dev docs (scikit-learn#13094)

Author: thomasjpfan

doc/developers/contributing.rst

@@ -115,7 +115,7 @@ following rules before submitting:
    `new algorithm requirements
    <http://scikit-learn.org/stable/faq.html#what-are-the-inclusion-criteria-for-new-algorithms>`_.
 
--  If you are submitting a bug report, we strongly encourage you to follow the guidelines in 
+-  If you are submitting a bug report, we strongly encourage you to follow the guidelines in
    :ref:`filing_bugs`.
 
 .. _filing_bugs:
@@ -193,7 +193,7 @@ then submit a "pull request" (PR):
     account on the GitHub user account. For more details on how to fork a
     repository see `this guide <https://help.github.com/articles/fork-a-repo/>`_.
 
- 3. Clone your fork of the scikit-learn repo from your GitHub account to your 
+ 3. Clone your fork of the scikit-learn repo from your GitHub account to your
     local disk::
 
         $ git clone [email protected]:YourLogin/scikit-learn.git
@@ -203,7 +203,7 @@ then submit a "pull request" (PR):
 
         $ pip install --editable .
 
-    for more details about advanced installation, see the 
+    for more details about advanced installation, see the
     :ref:`install_bleeding_edge` section.
 
  5. Create a branch to hold your development changes::
@@ -342,8 +342,8 @@ rules before submitting a pull request:
   documentation, see the :ref:`contribute_documentation` section.
 
 * The documentation should also include expected time and space complexity
-  of the algorithm and scalability, e.g. "this algorithm can scale to a 
-  large number of samples > 100000, but does not scale in dimensionality: 
+  of the algorithm and scalability, e.g. "this algorithm can scale to a
+  large number of samples > 100000, but does not scale in dimensionality:
   n_features is expected to be lower than 100".
 
 You can also check for common programming errors with the following tools:
@@ -390,7 +390,7 @@ Continuous Integration (CI)
 * Travis is used for testing on Linux platforms
 * Appveyor is used for testing on Windows platforms
 * CircleCI is used to build the docs for viewing, for linting with flake8, and
-    for testing with PyPy on Linux 
+    for testing with PyPy on Linux
 
 Please note that if one of the following markers appear in the latest commit
 message, the following actions are taken.
@@ -489,7 +489,7 @@ documents live in the source code repository under the ``doc/`` directory.
 
 You can edit the documentation using any text editor, and then generate the
 HTML output by typing ``make html`` from the ``doc/`` directory. Alternatively,
-``make`` can be used to quickly generate the documentation without the example 
+``make`` can be used to quickly generate the documentation without the example
 gallery. The resulting HTML files will be placed in ``_build/html/stable`` and are viewable
 in a web browser. See the ``README``file in the ``doc/`` directory for more information.
 
@@ -548,10 +548,10 @@ details, and give intuition to the reader on what the algorithm does.
 
 Basically, to elaborate on the above, it is best to always
 start with a small paragraph with a hand-waving explanation of what the
-method does to the data. Then, it is very helpful to point out why the feature is 
+method does to the data. Then, it is very helpful to point out why the feature is
 useful and when it should be used - the latter also including "big O"
-(:math:`O\left(g\left(n\right)\right)`) complexities of the algorithm, as opposed 
-to just *rules of thumb*, as the latter can be very machine-dependent. If those 
+(:math:`O\left(g\left(n\right)\right)`) complexities of the algorithm, as opposed
+to just *rules of thumb*, as the latter can be very machine-dependent. If those
 complexities are not available, then rules of thumb may be provided instead.
 
 Secondly, a generated figure from an example (as mentioned in the previous
@@ -1225,6 +1225,14 @@ Optional Arguments
 In iterative algorithms, the number of iterations should be specified by
 an integer called ``n_iter``.
 
+Pairwise Attributes
+^^^^^^^^^^^^^^^^^^^
+
+An estimator that accept ``X`` of shape ``(n_samples, n_samples)`` and defines
+a :term:`_pairwise` property equal to ``True`` allows for cross-validation of
+the dataset, e.g. when ``X`` is a precomputed kernel matrix. Specifically,
+the :term:`_pairwise` property is used by ``utils.metaestimators._safe_split``
+to slice rows and columns.
 
 Rolling your own estimator
 ==========================