Documentation/git-merge.txt: weaken warning about uncommited changes

Commit 35d2fff introduced 'git merge --abort' as a synonym to 'git reset
--merge', and added some failing tests in t7611-merge-abort.sh (search
'###' in this file) showing that 'git merge --abort' could not always
recover the pre-merge state.

Still, in many cases, 'git merge --abort' just works, and it is usually
considered that the ability to start a merge with uncommited changes is
an important property of Git.

Weaken the warning by discouraging only merge with /non-trivial/
uncommited changes.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-merge.txt

@@ -56,8 +56,8 @@ especially if those changes were further modified after the merge
 was started), 'git merge --abort' will in some cases be unable to
 reconstruct the original (pre-merge) changes. Therefore:
 
-*Warning*: Running 'git merge' with uncommitted changes is
-discouraged: while possible, it leaves you in a state that is hard to
+*Warning*: Running 'git merge' with non-trivial uncommitted changes is
+discouraged: while possible, it may leave you in a state that is hard to
 back out of in the case of a conflict.