[LangRef] clarify the meaning of noimplicitfloat

Adds some more text to the documentation for the noimplicitfloat
function attribute. Hopefully, this makes it clearer what
qualifies an implicit vs. explicit float, without becoming overly
long or going into target-specific details.

Reviewed By: rnk, craig.topper

Differential Revision: https://reviews.llvm.org/D104061

llvm/docs/LangRef.rst

@@ -1635,7 +1635,11 @@ example:
     memory on it's behalf.  As a result, perhaps surprisingly, a ``nofree``
     function can return a pointer to a previously deallocated memory object.
 ``noimplicitfloat``
-    This attributes disables implicit floating-point instructions.
+    Disallows implicit floating-point code. This inhibits optimizations that
+    use floating-point code and floating-point/SIMD/vector registers for
+    operations that are not nominally floating-point. LLVM instructions that
+    perform floating-point operations or require access to floating-point
+    registers may still cause floating-point code to be generated.
 ``noinline``
     This attribute indicates that the inliner should never inline this
     function in any situation. This attribute may not be used together