Although the fact that some characters are illegal in derivation names
was referenced in some parts of the docs, the actual list was not
documented anywhere, and the only source of truth seems to be the
[store source code](lix/libstore/path.cc).
This adds an admonition in the `derivation` docs that specifies which
characters and strings are actually allowed (especially since some
of them can be somewhat surprising, like the string "..-foo" not
being a valid name because of `..`).
Change-Id: Ibc4b4a141e262c21306ce9c0392f92cf0abe610a
blokyk
526bcac44a