docs: Add pointer to cmake caches for PGO

Also add a link to end-user PGO documentation.

Differential Revision: https://reviews.llvm.org/D92768

llvm/docs/HowToBuildWithPGO.rst

@@ -12,6 +12,14 @@ decrease overall compile time by 20%.
 This guide walks you through how to build Clang with PGO, though it also applies
 to other subprojects, such as LLD.
 
+If you want to build other software with PGO, see the `end-user documentation
+for PGO <https://clang.llvm.org/docs/UsersManual.html#profile-guided-optimization>`_.
+
+
+Using preconfigured CMake caches
+================================
+
+See https://llvm.org/docs/AdvancedBuilds.html#multi-stage-pgo
 
 Using the script
 ================
@@ -62,8 +70,8 @@ This is configurable in the script itself.
 Building Clang with PGO
 =======================
 
-If you prefer to not use the script, this briefly goes over how to build
-Clang/LLVM with PGO.
+If you prefer to not use the script or the cmake cache, this briefly goes over
+how to build Clang/LLVM with PGO.
 
 First, you should have at least LLVM, Clang, and compiler-rt checked out
 locally.

llvm/utils/collect_and_build_with_pgo.py

@@ -10,6 +10,9 @@
 
 This is a total of four clean builds of clang (by default). This may take a
 while. :)
+
+This scripts duplicates https://llvm.org/docs/AdvancedBuilds.html#multi-stage-pgo
+Eventually, it will be updated to instead call the cmake cache mentioned there.
 """
 
 import argparse