x/pkgsite: support company-specific doc annotations

[bradfitz]

Google internally runs a godoc server serving docs for the standard library + all of
Google $GOPATH (all internal Google Go code).

In many cases, we'd like to mention the Google-specific libraries or ways of doing
things from the standard library docs.

It would be nice if there were a file we could maintain that appended company-specific
doc annotations to the standard library on at least:

-- packages
-- funcs
-- types
-- specific fields on struct types

[adg]

Comment 1:

We could also just modify the source tree internally at Google, rather than adding a
bunch of hooks to godoc.

[bradfitz]

Comment 2:

That's a possibility.
But doing it naively with patch files that would go stale over time as the standard
library changes would be a pain, so we'd want to not use diff+patch and an go/*-based
AST patcher instead. Once we have that, the godoc hooks aren't much more work and could
benefit other companies. Google's not unique in this regard.

[adg]

Comment 3:

A general tool for patching doc comments would also be generally useful. :-)
Either way it's done, the source format ("add this annotation to this doc comment") will
be the same.