unsafe: document valid uses of Pointer
unsafe: document valid uses of Pointer

Add docs for valid uses of Pointer.
Then document change made for #13372 in CL 18584.

Fixes #8994.

Change-Id: Ifba71e5aeafd11f684aed0b7ddacf3c8ec07c580
Reviewed-on: https://go-review.googlesource.com/18640
Reviewed-by: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
Reviewed-by: Rick Hudson <838bd060584c511f21c75c69ea04b7b120d86af0@golang.org>

cloudflare/go,linux-on-ibm-z/go,miolini/go,josharian/go,Samurais/go,josharian/go,miolini/go,miolini/go,sndnvaps/go,binarycrusader/go,polyverse-security/go,momchil-velikov/go,suncycheng/go,sndnvaps/go,momchil-velikov/go,4ad/go,suncycheng/go,theif519/go,cloudflare/go,Samurais/go,golang/go,CAFxX/go,mwhudson/go,theif519/go,codestation/go,ImJasonH/go,Cofyc/go,binarycrusader/go,4ad/go,golang/go,golang/go,momchil-velikov/go,sndnvaps/go,goodlang/good,polyverse-security/go,Samurais/go,Endika/go,vanloswang/go,suncycheng/go,Endika/go,ganboing/go-esx,Samurais/go,akutz/go,ganboing/go-esx,ganboing/go-esx,josharian/go,nawawi/go,Cofyc/go,Endika/go,Cofyc/go,binarycrusader/go,ImJasonH/go,linux-on-ibm-z/go,ImJasonH/go,josharian/go,codestation/go,CAFxX/go,akutz/go,nawawi/go,linux-on-ibm-z/go,nawawi/go,mwhudson/go,nawawi/go,goodlang/good,momchil-velikov/go,cloudflare/go,josharian/go,Samurais/go,CAFxX/go,cloudflare/go,binarycrusader/go,josharian/go,derekmarcotte/go,golang/go,golang/go,momchil-velikov/go,derekmarcotte/go,vanloswang/go,nawawi/go,golang/go,vsekhar/elastic-go,vsekhar/elastic-go,polyverse-security/go,mwhudson/go,4ad/go,ImJasonH/go,derekmarcotte/go,vanloswang/go,derekmarcotte/go,CAFxX/go,mwhudson/go,akutz/go,nawawi/go,Endika/go,derekmarcotte/go,suncycheng/go,nawawi/go,mwhudson/go,Cofyc/go,sndnvaps/go,binarycrusader/go,ganboing/go-esx,theif519/go,akutz/go,linux-on-ibm-z/go,mwhudson/go,sndnvaps/go,derekmarcotte/go,Endika/go,sndnvaps/go,4ad/go,goodlang/good,mwhudson/go,vsekhar/elastic-go,polyverse-security/go,akutz/go,Endika/go,vanloswang/go,cloudflare/go,CAFxX/go,Endika/go,suncycheng/go,CAFxX/go,josharian/go,linux-on-ibm-z/go,Cofyc/go,goodlang/good,polyverse-security/go,miolini/go,theif519/go,Cofyc/go,linux-on-ibm-z/go,codestation/go,goodlang/good,vsekhar/elastic-go,miolini/go,ImJasonH/go,polyverse-security/go,akutz/go,Cofyc/go,4ad/go,ganboing/go-esx,miolini/go,4ad/go,mwhudson/go,vanloswang/go,binarycrusader/go,momchil-velikov/go,sndnvaps/go,nawawi/go,theif519/go,vanloswang/go,miolini/go,ganboing/go-esx,cloudflare/go,vsekhar/elastic-go,theif519/go,theif519/go,4ad/go,christopher-henderson/Go,codestation/go,polyverse-security/go,miolini/go,vsekhar/elastic-go,vanloswang/go,cloudflare/go,codestation/go,suncycheng/go,golang/go,goodlang/good,ganboing/go-esx,ImJasonH/go,ImJasonH/go,Cofyc/go,josharian/go,ImJasonH/go,Samurais/go,cloudflare/go,momchil-velikov/go,vsekhar/elastic-go,momchil-velikov/go,linux-on-ibm-z/go,vsekhar/elastic-go,derekmarcotte/go,CAFxX/go,derekmarcotte/go,ganboing/go-esx,goodlang/good,Endika/go,codestation/go,CAFxX/go,akutz/go,golang/go,akutz/go,goodlang/good,vanloswang/go,suncycheng/go,theif519/go,linux-on-ibm-z/go,Samurais/go,4ad/go,codestation/go,codestation/go,polyverse-security/go,suncycheng/go,sndnvaps/go,binarycrusader/go,Samurais/go,binarycrusader/go