Doc: cover multiple contains/no-contains filters

Update documentation for "git branch", "git for-each-ref" and "git tag"
with notes explaining what happens when passed multiple --contains or
--no-contains filters.

This behavior is useful to document prior to enabling multiple
merged/no-merged filters, in order to demonstrate consistent behavior
between merged/no-merged and contains/no-contains filters.

Signed-off-by: Aaron Lipman <alipman88@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/filters.txt

@@ -0,0 +1,3 @@
+When combining multiple `--contains` and `--no-contains` filters, only
+references that contain at least one of the `--contains` commits and
+contain none of the `--no-contains` commits are shown.

Documentation/git-branch.txt

@@ -370,6 +370,8 @@ serve four related but different purposes:
 - `--no-merged` is used to find branches which are candidates for merging
   into HEAD, since those branches are not fully contained by HEAD.
 
+include::filters.txt[]
+
 SEE ALSO
 --------
 linkgit:git-check-ref-format[1],

Documentation/git-for-each-ref.txt

@@ -408,6 +408,11 @@ Note also that multiple copies of an object may be present in the object
 database; in this case, it is undefined which copy's size or delta base
 will be reported.
 
+NOTES
+-----
+
+include::filters.txt[]
+
 SEE ALSO
 --------
 linkgit:git-show-ref[1]

Documentation/git-tag.txt

@@ -377,6 +377,11 @@ $ GIT_COMMITTER_DATE="2006-10-02 10:31" git tag -s v1.0.1
 
 include::date-formats.txt[]
 
+NOTES
+-----
+
+include::filters.txt[]
+
 SEE ALSO
 --------
 linkgit:git-check-ref-format[1].