io: document that ReadAtLeast and ReadFull can drop errors
io: document that ReadAtLeast and ReadFull can drop errors

Add a note that if an error is returned after having read
at least the minimum no. of bytes, the error is set to nil.

Fixes #20477

Change-Id: I75ba5ee967be3ff80249e40d459da4afeeb53463
Reviewed-on: https://go-review.googlesource.com/102459
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

akutz/go,Cofyc/go,Cofyc/go,cloudflare/go,golang/go,akutz/go,CAFxX/go,golang/go,golang/go,CAFxX/go,Cofyc/go,cloudflare/go,josharian/go,nawawi/go,cloudflare/go,josharian/go,golang/go,cloudflare/go,golang/go,cloudflare/go,Cofyc/go,CAFxX/go,nawawi/go,Cofyc/go,akutz/go,nawawi/go,nawawi/go,akutz/go,Cofyc/go,josharian/go,akutz/go,CAFxX/go,josharian/go,nawawi/go,cloudflare/go,nawawi/go,Cofyc/go,nawawi/go,golang/go,akutz/go,nawawi/go,cloudflare/go,josharian/go,CAFxX/go,Cofyc/go,CAFxX/go,golang/go,cloudflare/go,akutz/go,CAFxX/go,akutz/go,golang/go,josharian/go,josharian/go,josharian/go,CAFxX/go