[Docs] Add HelpText for -mimplicit-float so it shows up in the webpage documentation. (llvm#153253)

There is no documentation for -mimplicit-float, -mno-implicit-float here
https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-mimplicit-float

I believe this is because the text is taken from the positive option
when there is a no- version. Add HelpText to the positive option to
hopefully fix this.

These options also affect vector and not just FP so having text here
that mentions vectors is helpful to users.

Author: topperc

clang/include/clang/Driver/Options.td

@@ -5589,7 +5589,8 @@ def mno_outline_atomics : Flag<["-"], "mno-outline-atomics">, Group<f_clang_Grou
   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 mimplicit_float : Flag<["-"], "mimplicit-float">, Group<m_Group>,
+  HelpText<"Generate implicit floating point or vector instructions">;
 def mrecip : Flag<["-"], "mrecip">, Group<m_Group>,
   Visibility<[ClangOption, CC1Option, FlangOption, FC1Option]>,
   HelpText<"Equivalent to '-mrecip=all'">;