clock_gettime: Catch up with the CLOCK_BOOTTIME changes

When CLOCK_BOOTTIME was made an alias for CLOCK_MONOTONIC, the man page
wasn't updated. Update to include the changed semantics (it was to be
better compatible with Linux). Clarify the details of these clocks.
Provide a reference to the UTC standard, and a clarifying remark
about leap second encoding (fun fact: POSIX provides no unique
value for these, nor definitive values to use).

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