cmd/asm: update package doc to describe "-p" option
cmd/asm: update package doc to describe "-p" option

Include a description of the assembler's "-p" command line option
in the package doc.

Fixes #54215.

Change-Id: I641abad573c37aa9447af6e8b84716093c2a2b70
Reviewed-on: https://go-review.googlesource.com/c/go/+/420900
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go