sync: delete a sentence from the Map docs
sync: delete a sentence from the Map docs

From Josh's comments on https://golang.org/cl/50310

Once I removed the "from the Go standard library" bit, the beginning
wasn't worth keeping. It also wasn't clear whether what it meant by
"cache contention". Processor caches, or user-level caches built with
sync.Map? It didn't seem worth clarifying and didn't convey any useful
information, so deleted.

Change-Id: Id1d76105a3081d0855f6a64540700932bb83d98e
Reviewed-on: https://go-review.googlesource.com/50632
Reviewed-by: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

codestation/go,josharian/go,christopher-henderson/Go,cloudflare/go,cloudflare/go,vsekhar/elastic-go,polyverse-security/go,codestation/go,polyverse-security/go,CAFxX/go,vsekhar/elastic-go,nawawi/go,codestation/go,cloudflare/go,Cofyc/go,josharian/go,codestation/go,nawawi/go,Cofyc/go,polyverse-security/go,polyverse-security/go,vsekhar/elastic-go,Cofyc/go,vsekhar/elastic-go,josharian/go,nawawi/go,CAFxX/go,polyverse-security/go,Cofyc/go,akutz/go,Cofyc/go,codestation/go,golang/go,akutz/go,cloudflare/go,golang/go,nawawi/go,vsekhar/elastic-go,josharian/go,golang/go,akutz/go,cloudflare/go,codestation/go,josharian/go,nawawi/go,golang/go,golang/go,nawawi/go,vsekhar/elastic-go,polyverse-security/go,CAFxX/go,CAFxX/go,Cofyc/go,Cofyc/go,codestation/go,josharian/go,golang/go,CAFxX/go,akutz/go,polyverse-security/go,josharian/go,cloudflare/go,CAFxX/go,nawawi/go,codestation/go,vsekhar/elastic-go,polyverse-security/go,nawawi/go,akutz/go,CAFxX/go,cloudflare/go,josharian/go,akutz/go,golang/go,Cofyc/go,cloudflare/go,golang/go,vsekhar/elastic-go,CAFxX/go,akutz/go,akutz/go