_content/doc: add additional pgo explanation

I was confused by this because it's listed in the "Compiler" section
- I assumed that the performance gains from PGO may have come from
compilation time.

Update the pgo blog post to mention the range of potential performance
improvements, which were noted in the 1.22 changelog.

Change-Id: I0d7cdd444cbb561eab20af35f3e923e7002d7eca
Reviewed-on: https://go-review.googlesource.com/c/website/+/564896
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>

_content/doc/go1.22.html

@@ -236,7 +236,7 @@ defer func() {
   <a href="/doc/pgo">Profile-guided Optimization (PGO)</a> builds
   can now devirtualize a higher proportion of calls than previously possible.
   Most programs from a representative set of Go programs now see between 2 and
-  14% improvement from enabling PGO.
+  14% improvement at runtime from enabling PGO.
 </p>
 
 <p><!-- https://go.dev/cl/528321 -->

_content/doc/pgo.md

@@ -21,7 +21,7 @@ For example, the compiler may decide to more aggressively inline functions which
 
 In Go, the compiler uses CPU pprof profiles as the input profile, such as from [runtime/pprof](https://pkg.go.dev/runtime/pprof) or [net/http/pprof](https://pkg.go.dev/net/http/pprof).
 
-As of Go 1.21, benchmarks for a representative set of Go programs show that building with PGO improves performance by around 2-7%.
+As of Go 1.22, benchmarks for a representative set of Go programs show that building with PGO improves performance by around 2-14%.
 We expect performance gains to generally increase over time as additional optimizations take advantage of PGO in future versions of Go.