Clarify documentation of 'vc-responsible-backend' wrt symlinks

* lisp/vc/vc.el (vc-responsible-backend): Clarify that symlinks
are not resolved when the VC backend is reported.

* doc/lispref/files.texi (Truenames): Document
'vc-responsible-backend'.  (Bug#23436)
* doc/emacs/maintaining.texi (Version Control Systems): Fix a
typo.

doc/emacs/maintaining.texi

@@ -223,7 +223,7 @@ basic editing operations under Bazaar.
 @cindex SRC
 @cindex src
 @item
-SRC (src) is RCS, reloaded - a specialized version-control system
+SRC (src) is RCS, reloaded---a specialized version-control system
 designed for single-file projects worked on by only one person.  It
 allows multiple files with independent version-control histories to
 exist in one directory, and is thus particularly well suited for

doc/lispref/files.texi

@@ -1138,6 +1138,21 @@ compares the truenames of the two directories.  If @var{dir} does not
 name an existing directory, the return value is @code{nil}.
 @end defun
 
+@defun vc-responsible-backend file
+This function determines the responsible VC backend of the given
+@var{file}.  For example, if @file{emacs.c} is a file tracked by Git,
+@w{@code{(vc-responsible-backend "emacs.c")}} returns @samp{Git}.
+Note that if @var{file} is a symbolic link,
+@code{vc-responsible-backend} will not resolve it---the backend of the
+symbolic link file itself is reported.  To get the backend VC of the
+file to which @var{file} refers, wrap @var{file} with a symbolic link
+resolving function such as @code{file-chase-links}:
+
+@smallexample
+(vc-responsible-backend (file-chase-links "emacs.c"))
+@end smallexample
+@end defun
+
 @node File Attributes
 @subsection File Attributes
 @cindex file attributes

lisp/vc/vc.el

@@ -959,7 +959,11 @@ use."
 If FILE is already registered, return the
 backend of FILE.  If FILE is not registered, then the
 first backend in `vc-handled-backends' that declares itself
-responsible for FILE is returned."
+responsible for FILE is returned.
+
+Note that if FILE is a symbolic link, it will not be resolved --
+the responsible backend system for the symbolic link itself will
+be reported."
   (or (and (not (file-directory-p file)) (vc-backend file))
       (catch 'found
 	;; First try: find a responsible backend.  If this is for registration,