Documentation/git-merge.txt: Partial rewrite of How Merge Works

Documentation/git-merge.txt: Partial rewrite of How Merge Works

The git-merge documentation's "HOW MERGE WORKS" section is confusingly
composed and actually omits the most interesting part, the merging of
the arguments into HEAD itself, surprisingly not actually mentioning
the fast-forward merge anywhere.

This patch replaces the "[NOTE]" screenful of highly technical details
by a single sentence summing up the interesting information, and instead
explains how are the arguments compared with HEAD and the three possible
inclusion states that are named "Already up-to-date", "Fast-forward"
and "True merge". It also makes it clear that the rest of the section
talks only about the true merge situation, and slightly expands the
talk on solving conflicts.

Junio initiated the removal of the Note screenful altogether and
offered many stylistical fixes.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>