runtime: tweak doc for Goexit

Use singular form of panic and remove the unnecessary
'however', when comparing Goexit's behavior to 'a panic'
as well as what happens for deferred recovers with Goexit.

Change-Id: I3116df3336fa135198f6a39cf93dbb88a0e2f46e
Reviewed-on: https://go-review.googlesource.com/79755


Reviewed-by: Rob Pike <r@golang.org>