runtime: rename pageAlloc receiver

runtime: rename pageAlloc receiver

The history of pageAlloc using 's' as a receiver are lost to the depths
of time (perhaps it used to be called summary?), but it doesn't make
much sense anymore. Rename it to 'p'.

Generated with:

$ cd src/runtime
$ grep -R -b "func (s \*pageAlloc" . | awk -F : '{ print $1 ":#" $2+6 }' | xargs -n 1 -I {} env GOROOT=$(pwd)/../../ gorename -offset {} -to p -v
$ grep -R -b "func (s \*pageAlloc" . | awk -F : '{ print $1 ":#" $2+6 }' | xargs -n 1 -I {} env GOROOT=$(pwd)/../../ GOARCH=386 gorename -offset {} -to p -v
$ GOROOT=$(pwd)/../../ gorename -offset mpagecache.go:#2397 -to p -v

($2+6 to advance past "func (".)

Plus manual comment fixups.

Change-Id: I2d521a1cbf6ebe2ef6aae92e654bfc33c63d1aa9
Reviewed-on: https://go-review.googlesource.com/c/go/+/250517
Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>