cmd/compile: improve escape analysis explanation
cmd/compile: improve escape analysis explanation

No code changes, only revised comments in an attempt to make
escape analysis slightly less confusing.

Updates #23109.

Change-Id: I5ee6cea0946ced63f6210ac4484a088bcdd862fb
Reviewed-on: https://go-review.googlesource.com/121001
Run-TryBot: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

golang/go,CAFxX/go,golang/go,CAFxX/go,golang/go,akutz/go,CAFxX/go,akutz/go,cloudflare/go,nawawi/go,josharian/go,josharian/go,nawawi/go,cloudflare/go,akutz/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,akutz/go,CAFxX/go,akutz/go,akutz/go,CAFxX/go,golang/go,cloudflare/go,golang/go,josharian/go,nawawi/go,nawawi/go,josharian/go,CAFxX/go,akutz/go,nawawi/go,cloudflare/go,josharian/go,josharian/go,akutz/go,cloudflare/go,cloudflare/go,josharian/go,nawawi/go,nawawi/go,golang/go,nawawi/go,josharian/go,golang/go