About page text, first pass.

Merges [30821] to the 4.1 branch.

props johnbillion, helen, markjaquith, nacin.
see #30435.

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Rename `(get_)the_pagination()` to `(get_)the_posts_pagination()` for clarity.

Merges [30823] to the 4.1 branch

See #29808
Props obenland

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Switch to `(get_)the_posts_pagination()` in Twenty Fifteen.

Merges [30824] to the 4.1 branch.

See #29808
Props obenland

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4.1 RC1

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