docs: move core.excludesFile from git-add to gitignore

A section in "git add" documentation mentions core.excludesFile and
explains how it works, but this is not specific to the command.

Move this description to gitignore.txt to be more generic.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-add.txt

@@ -193,15 +193,6 @@ for "git add --no-all <pathspec>...", i.e. ignored removed files.
 	for command-line options).
 
 
-CONFIGURATION
--------------
-
-The optional configuration variable `core.excludesFile` indicates a path to a
-file containing patterns of file names to exclude from git-add, similar to
-$GIT_DIR/info/exclude.  Patterns in the exclude file are used in addition to
-those in info/exclude.  See linkgit:gitignore[5].
-
-
 EXAMPLES
 --------
 

Documentation/gitignore.txt

@@ -132,6 +132,14 @@ full pathname may have special meaning:
  - Other consecutive asterisks are considered regular asterisks and
    will match according to the previous rules.
 
+CONFIGURATION
+-------------
+
+The optional configuration variable `core.excludesFile` indicates a path to a
+file containing patterns of file names to exclude, similar to
+`$GIT_DIR/info/exclude`.  Patterns in the exclude file are used in addition to
+those in `$GIT_DIR/info/exclude`.
+
 NOTES
 -----