STM32: Add comments for HAL_InitTick function

[bcostm]

Description

Add more comments for the HAL_InitTick function to explain this function is not intended to configure the SysTick but rather a HW timer used for timeouts only. I hope it is clearer, otherwise tell me...

Pull request type

[X] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2369
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7225/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2002
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7225/

[mbed-ci]

Test : SUCCESS

Build number : 2158
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7225/2158

[cmonr]

/morph uvisor-test