PortOut.h: Documentation improvements only

[TacoGrandeTX]

Description

Improve function and parameter descriptions

Pull request type

[ ] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[x] Docs update
[ ] Test update
[ ] Breaking change

[NirSonnenschein]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3353
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8421/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2987
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8421/

[mbed-ci]

Test : SUCCESS

Build number : 3158
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8421/3158

[cmonr]

/morph mbed2-build