[FPEnv][LangRef] Update doc for strictfp attribute

[kpn]

Summary

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

[arsenm]

I'd say this is already implied by the current wording, but doesn't hurt to be explicit

[arsenm]

Though I am wondering if there's any real reason to require anything of an indirect call site

[kpn]

In D148138#4265164, @arsenm wrote:

Though I am wondering if there's any real reason to require anything of an indirect call site

What if an optimization propagates a constant value into an indirect call? That would make it possible for the indirect call to become direct.

llvm/docs/LangRef.rst

 example, a series of FP operations that each may raise exceptions may be
 vectorized into a single instruction that raises each unique exception a single
 time.
 
 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.
 
 '``llvm.experimental.constrained.fadd``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Syntax: