Add a constraint on the requests managed by a mirror

[meox]

No description provided.

[mlugg]

This is unnecessary, as the constraint you're documenting already exists through a previously-listed rule:

• Invalid accesses, for instance to malformed filenames, may cause a 404 Not Found response.

Perhaps this could do with clarification. The intention here is that mirrors are only required to serve requests for filenames which match the tarball name schema, which consists of any of these:

zig-VERSION.EXTENSION
zig-bootstrap-VERSION.EXTENSION
zig-ARCH-OS-VERSION.EXTENSION

I'll push a quick clarification for this myself.

[mlugg]

72f4e4b clarifies that mirrors may 404 on invalid filenames and sets out more precisely what a valid filename looks like.