[clang][docs][RISCV] Pre-pend the HelpText for -mrvv-vector-bits into the DocBrief.

The DocBrief is used to generate the webpage description of the
optio. The current text only talks about the possible values, but
not what the option does.

Author: topperc

clang/include/clang/Driver/Options.td

@@ -5056,9 +5056,10 @@ def mrvv_vector_bits_EQ : Joined<["-"], "mrvv-vector-bits=">, Group<m_Group>,
   Visibility<[ClangOption, FlangOption]>,
   HelpText<"Specify the size in bits of an RVV vector register">,
   DocBrief<!strconcat(
-    "Defaults to the vector length agnostic value of \"scalable\". "
-    "Accepts power of 2 values between 64 and 65536. Also accepts "
-    "\"zvl\" to use the value implied by -march/-mcpu.",
+    "Specify the size in bits of an RVV vector register. Defaults to the "
+    "vector length agnostic value of \"scalable\". Accepts power of 2 values "
+    "between 64 and 65536. Also accepts \"zvl\" to use the value implied by "
+    "-march/-mcpu.",
     !cond(
       // Flang does not set the preprocessor define.
       !eq(GlobalDocumentation.Program, "Flang") : "",