Various improvements to conditionalCreate/Mediation discoverability and uniformity

[emlun]

Various opportunities for improvement I noticed while reviewing PR #1951, but some of this is beyond the scope of that PR. We should merge #1951 first, then this PR scope will be much smaller as most of these commits will have been merged already with #1951.

• Reword "This operation" to "Any operation" in get() too, for uniformity with the new language in create().
• Rephrase and shorten recommendation to check isConditionalMediationAvailable() or getClientCapabilities()
• Add cross-links between conditionalCreate and ~~conditionalMediation~~ conditionalGet client capabilities and where they are used
• Point out that ~~conditionalMediation~~ conditionalGet and isConditionalMediationAvailable() are equivalent

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Preview | Diff

[emlun]

Current diff compared to PR #1951: https://github.com/w3c/webauthn/pull/2046/files/aca236f39be577276a7ff962badc33ad6f0d41fb..1df325885582da74f580a4276fc171a707fc655d

[pascoej]

Thank you!