runtime: clarify runtime.GC blocking behavior
The runtime.GC documentation was rewritten in
df2809f to make it clear
that it blocks until GC is complete, but the re-rewrite in
ed9a4c9 and
e28a679 lost this property when clarifying that it may also block the
entire program and not just the caller.
Try to arrive at wording that conveys both of these properties.
Change-Id: I1e255322aa28a21a548556ecf2a44d8d8ac524ef
Reviewed-on: https://go-review.googlesource.com/12392
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>