add -q in help info and doc

gdb/doc/gdb.texinfo

@@ -885,10 +885,11 @@ This will cause @code{@value{GDBP}} to debug @code{gcc}, and to set
 @code{gcc}'s command-line arguments (@pxref{Arguments}) to @samp{-O2 -c foo.c}.
 
 You can run @code{@value{GDBP}} without printing the front material, which describes
-@value{GDBN}'s non-warranty, by specifying @code{-silent}:
+@value{GDBN}'s non-warranty, by specifying @code{--silent}
+(or @code{-q}/@code{--quiet}):
 
 @smallexample
-@value{GDBP} -silent
+@value{GDBP} --silent
 @end smallexample
 
 @noindent

gdb/main.c

@@ -1173,7 +1173,8 @@ Output and user interface control:\n\n\
   fputs_unfiltered (_("\
   --dbx              DBX compatibility mode.\n\
   --xdb              XDB compatibility mode.\n\
-  --quiet            Do not print version number on startup.\n\n\
+  -q, --quiet, --silent\n\
+                     Do not print version number on startup.\n\n\
 "), stream);
   fputs_unfiltered (_("\
 Operating modes:\n\n\