[doc][RISCV] Add XSfvfexp* and XSfvfexpa* into RISCVUsage.rst (#166198)

They were introduced in #164349

Author: mshockwave

llvm/docs/RISCVUsage.rst

@@ -406,6 +406,12 @@ The current vendor extensions supported are:
 ``XSfvcp``
   LLVM implements `version 1.1.0 of the SiFive Vector Coprocessor Interface (VCIX) Software Specification <https://sifive.cdn.prismic.io/sifive/Zn3m1R5LeNNTwnLS_vcix-spec-software-v1p1.pdf>`__ by SiFive.  All instructions are prefixed with `sf.vc.` as described in the specification, and the riscv-toolchain-convention document linked above.
 
+``Xsfvfexp16e``, ``Xsfvfbfexp16e``, and ``Xsfvfexp32e``
+  LLVM implements `version 0.5 of the Vector Exponential Extension Specification <https://www.sifive.com/document-file/exponential-function-instruction-xsfvfexp32e-xsfvf>`__ by SiFive. All instructions are prefixed with `sf.` as described in the specification linked above.
+
+``Xsfvfexpa`` and ``Xsfvfexpa64e``
+  LLVM implements `version 0.2 of the Vector Exponential Approximation Extension Specification <https://www.sifive.com/document-file/exponential-approximation-instruction-xsfvfexpa-ex>`__ by SiFive. All instructions are prefixed with `sf.` as described in the specification linked above.
+
 ``XSfvqmaccdod``, ``XSfvqmaccqoq``
   LLVM implements `version 1.1.0 of the SiFive Int8 Matrix Multiplication Extensions Specification <https://sifive.cdn.prismic.io/sifive/1a2ad85b-d818-49f7-ba83-f51f1731edbe_int8-matmul-spec.pdf>`__ by SiFive.  All instructions are prefixed with `sf.` as described in the specification linked above.