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 <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Than McIntosh <thanm@google.com>

src/cmd/asm/doc.go

@@ -37,6 +37,8 @@ Flags:
 		Write symbol ABI information to output file. Don't assemble.
 	-o file
 		Write output to file. The default is foo.o for /a/b/c/foo.s.
+	-p pkgpath
+		Set expected package import to pkgpath.
 	-shared
 		Generate code that can be linked into a shared library.
 	-spectre list