gas: correct .bss documentation for non-ELF

Only ELF permits the specification of a subsection, and even there not
consistently: csky, mcore, and spu handle .bss similar to .lcomm.

gas/doc/as.texi

@@ -4456,7 +4456,12 @@ Some machine configurations provide additional directives.
 * Asciz::                       @code{.asciz "@var{string}"}@dots{}
 * Attach_to_group::             @code{.attach_to_group @var{name}}
 * Balign::                      @code{.balign [@var{abs-expr}[, @var{abs-expr}]]}
+@ifset ELF
 * Bss::                         @code{.bss @var{subsection}}
+@end ifset
+@ifclear ELF
+* Bss::                         @code{.bss}
+@end ifclear
 * Bundle directives::           @code{.bundle_align_mode @var{abs-expr}}, etc
 * Byte::                        @code{.byte @var{expressions}}
 * CFI directives::		@code{.cfi_startproc [simple]}, @code{.cfi_endproc}, etc.
@@ -4801,14 +4806,20 @@ filled in with the value 0x368d (the exact placement of the bytes depends upon
 the endianness of the processor).  If it skips 1 or 3 bytes, the fill value is
 undefined.
 
+@ifset ELF
 @node Bss
 @section @code{.bss @var{subsection}}
+@end ifset
+@ifclear ELF
+@node Bss
+@section @code{.bss}
+@end ifclear
 @cindex @code{bss} directive
 
 @code{.bss} tells @command{@value{AS}} to assemble the following statements
 onto the end of the bss section.
 @ifset ELF
-For ELF based targets an optional @var{subsection} expression (which must
+For most ELF based targets an optional @var{subsection} expression (which must
 evaluate to a positive integer) can be provided.  In this case the statements
 are appended to the end of the indicated bss subsection.
 @end ifset