libmd: export and document *Fd/*FdChunk interfaces

[kevans]

Summary

Fixes: de13c2427d6c ("libmd: introduce functions that operate on an fd")

[debdrup]

I think you should add the "nearly" qualifier to identical, as saying they're identical and then mentioning an exception seems contradictory to me.

Otherwise, the syntax of the mdoc looks fine, can't speak for the code.

[emaste]

"...are identical, except..." is fine. If it weren't for the exception they would be identical, not nearly identical.

But if we really want to avoid the phrase something like "perform the same function as" or "are equivalent to" would do.

[pauamma_gundo.com]

Nothing un-Englishy jumps out to me.

[bcr]

Manpage looks good. Thanks!