[FPEnv][LangRef] Update doc for strictfp attribute

Based on the direction of IR Verifier changes in D146845, this documentation
needs to be updated.

Differential Revision: https://reviews.llvm.org/D148138

Author: kpneal

llvm/docs/LangRef.rst

@@ -23467,7 +23467,9 @@ Proper :ref:`function attributes <fnattrs>` usage is required for the
 constrained intrinsics to function correctly.
 
 All function *calls* done in a function that uses constrained floating
-point intrinsics must have the ``strictfp`` attribute.
+point intrinsics must have the ``strictfp`` attribute either on the
+calling instruction or on the declaration or definition of the function
+being called.
 
 All function *definitions* that use constrained floating point intrinsics
 must have the ``strictfp`` attribute.