x86: generalize gas documentation for disabling of ISA extensions

As of commit ae89daecb1 ("x86: generalize disabling of sub-
architectures") there's no arbitrary subset of ISAs which can also be
disabled. This should have been reflected in documentation right away.
Since I failed to do so, correct this now.

gas/doc/c-i386.texi

@@ -137,16 +137,9 @@ accept various extension mnemonics.  For example,
 @code{287},
 @code{387},
 @code{687},
-@code{no87},
-@code{no287},
-@code{no387},
-@code{no687},
 @code{cmov},
-@code{nocmov},
 @code{fxsr},
-@code{nofxsr},
 @code{mmx},
-@code{nommx},
 @code{sse},
 @code{sse2},
 @code{sse3},
@@ -155,18 +148,8 @@ accept various extension mnemonics.  For example,
 @code{sse4.1},
 @code{sse4.2},
 @code{sse4},
-@code{nosse},
-@code{nosse2},
-@code{nosse3},
-@code{nosse4a},
-@code{nossse3},
-@code{nosse4.1},
-@code{nosse4.2},
-@code{nosse4},
 @code{avx},
 @code{avx2},
-@code{noavx},
-@code{noavx2},
 @code{adx},
 @code{rdseed},
 @code{prfchw},
@@ -189,9 +172,7 @@ accept various extension mnemonics.  For example,
 @code{serialize},
 @code{tsxldtrk},
 @code{kl},
-@code{nokl},
 @code{widekl},
-@code{nowidekl},
 @code{hreset},
 @code{avx512f},
 @code{avx512cd},
@@ -213,37 +194,9 @@ accept various extension mnemonics.  For example,
 @code{avx512_bf16},
 @code{avx_vnni},
 @code{avx512_fp16},
-@code{noavx512f},
-@code{noavx512cd},
-@code{noavx512er},
-@code{noavx512pf},
-@code{noavx512vl},
-@code{noavx512bw},
-@code{noavx512dq},
-@code{noavx512ifma},
-@code{noavx512vbmi},
-@code{noavx512_4fmaps},
-@code{noavx512_4vnniw},
-@code{noavx512_vpopcntdq},
-@code{noavx512_vbmi2},
-@code{noavx512_vnni},
-@code{noavx512_bitalg},
-@code{noavx512_vp2intersect},
-@code{notdx},
-@code{noavx512_bf16},
-@code{noavx_vnni},
-@code{noavx512_fp16},
-@code{noenqcmd},
-@code{noserialize},
-@code{notsxldtrk},
 @code{amx_int8},
-@code{noamx_int8},
 @code{amx_bf16},
-@code{noamx_bf16},
 @code{amx_tile},
-@code{noamx_tile},
-@code{nouintr},
-@code{nohreset},
 @code{vmx},
 @code{vmfunc},
 @code{smx},
@@ -291,8 +244,8 @@ accept various extension mnemonics.  For example,
 @code{tlbsync},
 @code{svme} and
 @code{padlock}.
-Note that rather than extending a basic instruction set, the extension
-mnemonics starting with @code{no} revoke the respective functionality.
+Note that these extension mnemonics can be prefixed with @code{no} to revoke
+the respective (and any dependent) functionality.
 
 When the @code{.arch} directive is used with @option{-march}, the
 @code{.arch} directive will take precedent.
@@ -1559,6 +1512,9 @@ conditional jumps will be promoted when necessary to a two instruction
 sequence consisting of a conditional jump of the opposite sense around
 an unconditional jump to the target.
 
+Note that the sub-architecture specifiers (starting with a dot) can be prefixed
+with @code{no} to revoke the respective (and any dependent) functionality.
+
 Following the CPU architecture (but not a sub-architecture, which are those
 starting with a dot), you may specify @samp{jumps} or @samp{nojumps} to
 control automatic promotion of conditional jumps. @samp{jumps} is the