docs: document norm parameter for embedding scatter plots

[Zethson]

Summary

• norm was missing from the docstrings of sc.pl.embedding and related functions (umap, pca, tsne, embedding_density, ...) even though it's an accepted parameter, since doc_vbound_percentile (used by these functions) didn't document it while the separate doc_vboundnorm (used elsewhere) did.
• Added the norm entry to doc_vbound_percentile.

Closes #2116

[codecov]

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Additional details and impacted files

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Flag	Coverage Δ
hatch-test.low-vers	77.52% <ø> (?)
hatch-test.pre	79.16% <ø> (?)

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Files with missing lines	Coverage Δ
src/scanpy/plotting/legacy/_docs.py	100.00% <ø> (ø)