branch: improve documentation and naming of create_branch() parameters

branch: improve documentation and naming of create_branch() parameters

The documentation for 'create_branch()' was incomplete as it didn't say
what certain parameters were used for. Further a parameter name wasn't
very communicative.

So, add missing documentation for the sake of completeness and easy
reference. Also, rename the concerned parameter to make its name more
communicative.

Signed-off-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>