Basic documentation of -mrecip=... option

[tim.schmielau]

Summary

The documentation is still rather terse because it needs to fit
onto a single line of clang --help output.
But at least it lists what the user can specify and documents the
non-obvious syntax.

[craig.topper]

LGTM

[tim.schmielau]

Thank you Craig.
Can someone with commit access please land the patch for me? Thanks!

clang/include/clang/Driver/Options.td

def mno_fmv : Flag<["-"], "mno-fmv">, Group<f_clang_Group>, Flags<[CC1Option]>,
   HelpText<"Disable function multiversioning">;
 def moutline_atomics : Flag<["-"], "moutline-atomics">, Group<f_clang_Group>, Flags<[CC1Option]>,
   HelpText<"Generate local calls to out-of-line atomic operations">;
 def mno_outline_atomics : Flag<["-"], "mno-outline-atomics">, Group<f_clang_Group>, Flags<[CC1Option]>,
   HelpText<"Don't generate local calls to out-of-line atomic operations">;
 def mno_implicit_float : Flag<["-"], "mno-implicit-float">, Group<m_Group>,
   HelpText<"Don't generate implicit floating point or vector instructions">;
 def mimplicit_float : Flag<["-"], "mimplicit-float">, Group<m_Group>;
-def mrecip : Flag<["-"], "mrecip">, Group<m_Group>;
+def mrecip : Flag<["-"], "mrecip">, Group<m_Group>,
+  HelpText<"Equivalent to '-mrecip=all'">;
 def mrecip_EQ : CommaJoined<["-"], "mrecip=">, Group<m_Group>, Flags<[CC1Option]>,
+  HelpText<"Control use of approximate reciprocal and reciprocal square root instructions followed by <n> iterations of "
+           "Newton-Raphson refinement. "
+           "<value> = ( ['!'] ['vec-'] ('rcp'|'sqrt') [('h'|'s'|'d')] [':'<n>] ) | 'all' | 'default' | 'none'">,
   MarshallingInfoStringVector<CodeGenOpts<"Reciprocals">>;
 def mprefer_vector_width_EQ : Joined<["-"], "mprefer-vector-width=">, Group<m_Group>, Flags<[CC1Option]>,
   HelpText<"Specifies preferred vector width for auto-vectorization. Defaults to 'none' which allows target specific decisions.">,
   MarshallingInfoString<CodeGenOpts<"PreferVectorWidth">>;
 def mstack_protector_guard_EQ : Joined<["-"], "mstack-protector-guard=">, Group<m_Group>, Flags<[CC1Option]>,
   HelpText<"Use the given guard (global, tls) for addressing the stack-protector guard">,
   MarshallingInfoString<CodeGenOpts<"StackProtectorGuard">>;
 def mstack_protector_guard_offset_EQ : Joined<["-"], "mstack-protector-guard-offset=">, Group<m_Group>, Flags<[CC1Option]>,