Simplify headers and instructions
Simplify headers and instructions

There is confusion with having sub headings below Description.
This commit removes teh Description header completely and promotes
sub headings to full headings.

Some of the notes under each heading have also been updated to
provide better clarity.

andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,kjbracey-arm/mbed,mbedmicro/mbed,andcor02/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed