Fix help and documentation for inferior commands

This changes inferior.c to add Usage lines for all commands, and to
change how "metasyntactic variables" are written to conform to GNU
style.

While doing this I noticed that the manual doesn't document the
argument to "info inferiors", so I've added that as well.

ChangeLog
2018-04-27  Tom Tromey  <tom@tromey.com>

	* inferior.c (initialize_inferiors): Update help strings.

doc/ChangeLog
2018-04-27  Tom Tromey  <tom@tromey.com>

	* gdb.texinfo (Inferiors and Programs): Document argument to "info
	inferiors".

gdb/doc/gdb.texinfo

@@ -2708,9 +2708,12 @@ To find out what inferiors exist at any moment, use @w{@code{info
 inferiors}}:
 
 @table @code
-@kindex info inferiors
+@kindex info inferiors [ @var{id}@dots{} ]
 @item info inferiors
 Print a list of all inferiors currently being managed by @value{GDBN}.
+By default all inferiors are printed, but the argument @var{id}@dots{}
+-- a space separated list of inferior numbers -- can be used to limit
+the display to just the requested inferiors.
 
 @value{GDBN} displays for each inferior (in this order):
 

gdb/inferior.c

@@ -997,11 +997,14 @@ initialize_inferiors (void)
      initialize_current_architecture.  */
 
   add_info ("inferiors", info_inferiors_command,
-	    _("IDs of specified inferiors (all inferiors if no argument)."));
+	    _("Print a list of inferiors being managed.\n\
+Usage: info inferiors [ID]...\n\
+If IDs are specified, the list is limited to just those inferiors.\n\
+By default all inferiors are displayed."));
 
   c = add_com ("add-inferior", no_class, add_inferior_command, _("\
 Add a new inferior.\n\
-Usage: add-inferior [-copies <N>] [-exec <FILENAME>]\n\
+Usage: add-inferior [-copies N] [-exec FILENAME]\n\
 N is the optional number of inferiors to add, default is 1.\n\
 FILENAME is the file name of the executable to use\n\
 as main program."));
@@ -1013,22 +1016,25 @@ Usage: remove-inferiors ID..."));
 
   add_com ("clone-inferior", no_class, clone_inferior_command, _("\
 Clone inferior ID.\n\
-Usage: clone-inferior [-copies <N>] [ID]\n\
+Usage: clone-inferior [-copies N] [ID]\n\
 Add N copies of inferior ID.  The new inferior has the same\n\
 executable loaded as the copied inferior.  If -copies is not specified,\n\
 adds 1 copy.  If ID is not specified, it is the current inferior\n\
 that is cloned."));
 
   add_cmd ("inferiors", class_run, detach_inferior_command, _("\
-Detach from inferior ID (or list of IDS)."),
+Detach from inferior ID (or list of IDS).\n\
+Usage; detach inferiors ID..."),
 	   &detachlist);
 
   add_cmd ("inferiors", class_run, kill_inferior_command, _("\
-Kill inferior ID (or list of IDs)."),
+Kill inferior ID (or list of IDs).\n\
+Usage: kill inferiors ID..."),
 	   &killlist);
 
   add_cmd ("inferior", class_run, inferior_command, _("\
 Use this command to switch between inferiors.\n\
+Usage: inferior ID\n\
 The new inferior ID must be currently known."),
 	   &cmdlist);