Documentation/git-merge-tree.adoc: clarify the --merge-base option

Documentation/git-merge-tree.adoc: clarify the --merge-base option

The --merge-base option for merge-tree has a few slightly awkward
constructions or omissions:
  * Split the initial long sentence describing the option into two,
    making the instructions and the limitations clearer for readers.
  * Add context to the final sentence that might be obvious to some
    readers but isn't immediately obvious to all.
  * The discussion about lack of support for multiple merge bases
    simply leave folks wondering why that matters and could help or
    hurt.  Separate it out and add a brief explanation.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>