Add units/unit prefixes of frequency to doc-valid-idents

[ROMemories]

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.

[rustbot]

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[Centri3]

@bors r+

[bors]

:pushpin: Commit 62026c32288f86b4337dcf17bf4fabd15a140a92 has been approved by Centri3

It is now in the queue for this repository.

[bors]

:hourglass: Testing commit 62026c32288f86b4337dcf17bf4fabd15a140a92 with merge c2534dcc49b407930fa4e2205beb67eda55fce48...

[Centri3]

And actually yeah, putting these in test_units would've made more sense, specifically stuff like 3MHz. I won't hold it up for that though

[bors]

:sunny: Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test Approved by: Centri3 Pushing c2534dcc49b407930fa4e2205beb67eda55fce48 to master...

[ROMemories]

Thanks a lot for merging this!