doc: document --include/--exclude better

Problem reported by John Ruckstuhl (Bug#43782).
* doc/grep.texi (File and Directory Selection):
Document what happens if contradictory options are given,
or if no option matches a file name.
* doc/grep.in.1:

2 files changed, 15 insertions, 0 deletions

diff --git a/doc/grep.in.1 b/doc/grep.in.1
index 91c0bcac..7d5dfe5a 100644
--- a/doc/grep.in.1
+++ b/doc/grep.in.1
@@ -680,6 +680,17 @@ Search only files whose base name matches
 .I GLOB
 (using wildcard matching as described under
 .BR \-\^\-exclude ).
+If contradictory
+.B \-\^\-include
+and
+.B \-\^\-exclude
+options are given, the last matching one wins.
+If no
+.B \-\^\-include
+or
+.B \-\^\-exclude
+options match, a file is included unless the first such option is
+.BR \-\^\-include .
 .TP
 .BR \-r ", " \-\^\-recursive
 Read all files under each directory, recursively,
diff --git a/doc/grep.texi b/doc/grep.texi
index bb76ae10..0c0bb124 100644
--- a/doc/grep.texi
+++ b/doc/grep.texi
@@ -728,6 +728,10 @@ this is equivalent to the @samp{--binary-files=without-match} option.
 @cindex searching directory trees
 Search only files whose name matches @var{glob},
 using wildcard matching as described under @option{--exclude}.
+If contradictory @option{--include} and @option{--exclude} options are
+given, the last matching one wins.  If no @option{--include} or
+@option{--exclude} options match, a file is included unless the first
+such option is @option{--include}.
 
 @item -r
 @itemx --recursive