update function header
according coding style requirements:
The enum shall be declared as a typedef with the name
so update FieldIndex related code for IScsiDhcpExtractRootPath()
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@6945 6f19259b-4bc3-4df7-8a09-765794883524
change the old code style to comply with Doxgen format
[Impaction]
add comments for every function
[Reference Info]
add comments to achieve the highest standard of code quality matrix
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@5079 6f19259b-4bc3-4df7-8a09-765794883524