[docs][LangRef] Added minor update inside the frem. Fix : llvm#61653

Added minor update inside the `frem`. Fix : llvm#61653

Differential Revision: https://reviews.llvm.org/D146900

Author: aabhinavg1

llvm/docs/LangRef.rst

@@ -9586,7 +9586,13 @@ Overview:
 """""""""
 
 The '``frem``' instruction returns the remainder from the division of
-its two operands.
+its two operands. 
+
+.. note::
+
+The instruction is implemented as a call to libm's '``fmod``'
+for some targets, and using the instruction may thus require linking libm.
+
 
 Arguments:
 """"""""""