Adapt file-remote-p doc

* doc/lispref/files.texi (Magic File Names): Adapt file-remote-p.

* lisp/files.el (file-remote-p): Adapt docstring.

Author: albinus

doc/lispref/files.texi

@@ -3624,10 +3624,18 @@ ensure this principle is valid.
 
 @var{identification} specifies which part of the identifier shall be
 returned as string.  @var{identification} can be the symbol
-@code{method}, @code{user} or @code{host}; any other value is handled
-like @code{nil} and means to return the complete identifier string.
-In the example above, the remote @code{user} identifier string would
-be @code{root}.
+@code{method}, @code{user}, @code{host} or @code{localname}; any other
+value is handled like @code{nil} and means to return the complete
+identifier string.  In the example above, the remote @code{user}
+identifier string would be @code{root}.
+
+If the remote @var{file} does not contain a method, a user name, or a
+host name, the respective default value is returned.  The string
+returned for @var{identification} @code{localname} can differ depending
+on whether there is an existing connection.  File name handler specific
+implementations could support further @var{identification} symbols;
+@xref{Top, Tramp, Tramp, tramp}, for example, knows also the @code{hop}
+symbol.
 
 If @var{connected} is non-@code{nil}, this function returns @code{nil}
 even if @var{filename} is remote, if Emacs has no network connection

lisp/files.el

@@ -1298,9 +1298,14 @@ any that are missing.
 IDENTIFICATION can specify which part of the identification to
 return.  IDENTIFICATION can be the symbol `method', `user',
 `host', or `localname'.  Any other value is handled like nil and
-means to return the complete identification.  The string returned
-for IDENTIFICATION `localname' can differ depending on whether
-there is an existing connection.
+means to return the complete identification.
+
+If the remote FILE does not contain a method, a user name, or a host
+name, the respective default value is returned.  The string returned for
+IDENTIFICATION `localname' can differ depending on whether there is an
+existing connection.  File name handler specific implementations could
+support further IDENTIFICATION symbols; Tramp, for example, knows also
+the `hop' symbol.
 
 If CONNECTED is non-nil, return an identification only if FILE is
 located on a remote system and a connection is established to