Document how GDB searches for files when using -s, -e, and -se options

GDB's documentation of the 'file' command says:

    If you do not specify a directory and the file is not found in the
    GDB working directory, GDB uses the environment variable PATH as a
    list of directories to search, just as the shell does when looking
    for a program to run.

The same is true for files specified via commandline options -s, -e,
and -se.

This commit adds a cross reference to the file command for these options.

gdb/doc/gdb.texinfo

@@ -962,6 +962,11 @@ If @value{GDBN} has not been configured to included core file support,
 such as for most embedded targets, then it will complain about a second
 argument and ignore it.
 
+For the @samp{-s}, @samp{-e}, and @samp{-se} options, and their long
+form equivalents, the method used to search the file system for the
+symbol and/or executable file is the same as that used by the
+@code{file} command.  @xref{Files, ,file}.
+
 Many options have both long and short forms; both are shown in the
 following list.  @value{GDBN} also recognizes the long forms if you truncate
 them, so long as enough of the option is present to be unambiguous.