internal/asm/...: document packages and functions

[kortschak]

Currently the asm packages have zero documentation. There are four ways we can go with this:

1. Leave it as is.
2. Add a package-level stanza with an explanation of the absence of documentation and that users who are interested should look at the source.
3. Add package-level documentation and reflect the naive Go source for each function. The justification for this is that they functions are all trivial and they are intended as replacements for the Go.
4. Add package-level documentation and write English documentation.

My preferred options are 2 or 3.