[RISCV][docs] Explain Relaxation Default

As requested in follow-up comments on #108592.

Author: lenary

llvm/docs/RISCVUsage.rst

@@ -446,7 +446,7 @@ Some of the RISC-V psABI variants reserve ``gp`` (``x3``) for use as a "Global P
 To use this functionality, you need to be doing all of the following:
 * Use the ``medlow`` (aka ``small``) code model;
 * Not use the ``gp`` register for any other uses (some platforms use it for the shadow stack and others as a temporary -- as denoted by the ``Tag_RISCV_x3_reg_usage`` build attribute);
-* Compile your objects with Clang's ``-mrelax`` option, to enable relaxation annotations on relocatable objects;
+* Compile your objects with Clang's ``-mrelax`` option, to enable relaxation annotations on relocatable objects (this is the default, but ``-mno-relax`` disables these relaxation annotations);
 * Compile for a position-dependent static executable (not a shared library, and ``-fno-PIC`` / ``-fno-pic`` / ``-fno-pie``); and
 * Use LLD's ``--relax-gp`` option.