doc/gdb: mention GOTRACEBACK=crash
doc/gdb: mention GOTRACEBACK=crash

Also fix a couple of other errors.

Fixes #6877

Change-Id: I94c81c5847cc7b0adab19418e71687bc2ee7fe94
Reviewed-on: https://go-review.googlesource.com/34960
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

ImJasonH/go,josharian/go,linux-on-ibm-z/go,akutz/go,polyverse-security/go,vsekhar/elastic-go,nawawi/go,Cofyc/go,josharian/go,akutz/go,codestation/go,nawawi/go,vsekhar/elastic-go,ImJasonH/go,CAFxX/go,linux-on-ibm-z/go,polyverse-security/go,nawawi/go,Cofyc/go,vsekhar/elastic-go,Cofyc/go,CAFxX/go,codestation/go,cloudflare/go,polyverse-security/go,derekmarcotte/go,derekmarcotte/go,polyverse-security/go,cloudflare/go,cloudflare/go,golang/go,ImJasonH/go,CAFxX/go,codestation/go,josharian/go,akutz/go,CAFxX/go,linux-on-ibm-z/go,josharian/go,nawawi/go,cloudflare/go,derekmarcotte/go,golang/go,akutz/go,vsekhar/elastic-go,Cofyc/go,golang/go,ImJasonH/go,polyverse-security/go,vsekhar/elastic-go,derekmarcotte/go,Cofyc/go,linux-on-ibm-z/go,cloudflare/go,nawawi/go,vsekhar/elastic-go,golang/go,akutz/go,golang/go,derekmarcotte/go,golang/go,christopher-henderson/Go,derekmarcotte/go,codestation/go,cloudflare/go,codestation/go,ImJasonH/go,nawawi/go,ImJasonH/go,polyverse-security/go,josharian/go,nawawi/go,ImJasonH/go,CAFxX/go,codestation/go,Cofyc/go,linux-on-ibm-z/go,akutz/go,vsekhar/elastic-go,codestation/go,Cofyc/go,vsekhar/elastic-go,CAFxX/go,akutz/go,codestation/go,josharian/go,polyverse-security/go,CAFxX/go,josharian/go,golang/go,linux-on-ibm-z/go,linux-on-ibm-z/go,cloudflare/go,polyverse-security/go,Cofyc/go,linux-on-ibm-z/go,cloudflare/go,josharian/go,nawawi/go,CAFxX/go,ImJasonH/go,akutz/go,golang/go,derekmarcotte/go,derekmarcotte/go