vimdiff: add tool documentation

vimdiff: add tool documentation

Running 'git {merge,diff}tool --tool-help' now also prints usage
information about the vimdiff tool (and its variants) instead of just
its name.

Two new functions ('diff_cmd_help()' and 'merge_cmd_help()') have been
added to the set of functions that each merge tool (ie. scripts found
inside "mergetools/") can overwrite to provided tool specific
information.

Right now, only 'mergetools/vimdiff' implements these functions, but
other tools are encouraged to do so in the future, specially if they
take configuration options not explained anywhere else (as it is the
case with the 'vimdiff' tool and the new 'layout' option)

Note that the function 'show_tool_names', used in the implementation of
'git mergetool --tool-help', is also used in Documentation/Makefile to
generate the list of allowed values for the configuration variables
'{diff,merge}.{gui,}tool'. Adjust the rule so its output is an Asciidoc
"description list" instead of a plain list, with the tool name as the
item and the newly added tool description as the description.

In addition, a section has been added to
"Documentation/git-mergetool.txt" to explain the new "layout"
configuration option with examples.

Helped-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Fernando Ramos <greenfoo@u92.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>