cred: fix kernel-doc warnings in cred.h

Use the correct function parameter names, function names, or kernel-doc
format, and add function return comment sections to avoid kernel-doc
warnings:

Warning: include/linux/cred.h:43 function parameter 'gi' not described
 in 'get_group_info'
Warning: include/linux/cred.h:43 No description found for return value
 of 'get_group_info'
Warning: include/linux/cred.h:213 No description found for return value
 of 'get_cred_many'
Warning: include/linux/cred.h:260 function parameter '_cred' not described
 in 'put_cred_many'
Warning: include/linux/cred.h:260 expecting prototype for put_cred().
 Prototype was for put_cred_many() instead

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
[PM: subject tweak]
Signed-off-by: Paul Moore <paul@paul-moore.com>