Documentation: clarify use of .git{ignore,attributes} versus .git/info/*

gitignore patterns can be read from three different
files, while gitattributes can come from two files. Let's
provide some hints to the user about the differences and how
they are typically used.

Suggested by Toby Corkindale, but gratuitously reworded by Jeff King.

Signed-off-by: Toby Corkindale <toby.corkindale@rea-group.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitattributes.txt

@@ -63,6 +63,13 @@ path in question, and its parent directories (the further the
 directory that contains `.gitattributes` is from the path in
 question, the lower its precedence).
 
+If you wish to affect only a single repository (i.e., to assign
+attributes to files that are particular to one user's workflow), then
+attributes should be placed in the `$GIT_DIR/info/attributes` file.
+Attributes which should be version-controlled and distributed to other
+repositories (i.e., attributes of interest to all users) should go into
+`.gitattributes` files.
+
 Sometimes you would need to override an setting of an attribute
 for a path to `unspecified` state.  This can be done by listing
 the name of the attribute prefixed with an exclamation point `!`.

Documentation/gitignore.txt

@@ -38,6 +38,18 @@ precedence, the last matching pattern decides the outcome):
  * Patterns read from the file specified by the configuration
    variable 'core.excludesfile'.
 
+Which file to place a pattern in depends on how the pattern is meant to
+be used. Patterns which should be version-controlled and distributed to
+other repositories via clone (i.e., files that all developers will want
+to ignore) should go into a `.gitignore` file. Patterns which are
+specific to a particular repository but which do not need to be shared
+with other related repositories (e.g., auxiliary files that live inside
+the repository but are specific to one user's workflow) should go into
+the `$GIT_DIR/info/exclude` file.  Patterns which a user wants git to
+ignore in all situations (e.g., backup or temporary files generated by
+the user's editor of choice) generally go into a file specified by
+`core.excludesfile` in the user's `~/.gitconfig`.
+
 The underlying git plumbing tools, such as
 linkgit:git-ls-files[1] and linkgit:git-read-tree[1], read
 `gitignore` patterns specified by command-line options, or from