Add units/unit prefixes of frequency to doc-valid-idents
These units/unit prefixes often come up in the embedded world.
Should this PR also modify the `test_units` test? It seems only concerned with data units currently; should it also test frequency units?
changelog: [`doc_markdown`]: Add MHz, GHz, and THz to `doc-valid-idents`.
* "AccessKit" is a commonly used accessibility toolkit used in Rust GUIs.
* "CoreFoundation", "CoreGraphics", "CoreText" are frameworks on Apple OSes.
* "Direct2D", "Direct3D", "DirectWrite" are frameworks on Windows
* "PostScript" is a programming language and is mentioned when talking about
text and vector graphics.
* "OpenAL" is an audio framework / API.
* "OpenType" is a font format (TrueType is already mentioned).
* "WebRTC", "WebSocket", "WebTransport" are web networking technologies.
* "NetBSD" and "OpenBSD" are like the already included FreeBSD.
The OpenTelemetry project's name is all one word (see https://opentelemetry.io),
so currently triggers a false positive in the `doc_markdown` lint.
The project is increasing rapidly in popularity, so it seems like a worthy
contender for inclusion in the default `doc_valid_idents` configuration.
I've also moved the existing "OpenDNS" entry earlier in the list, to restore
the alphabetical ordering of that "Open*" row.
The docs changes were generated using `cargo collect-metadata`.
changelog: [`doc_markdown`]: Add `OpenTelemetry` to the default configuration as an allowed identifier
