Update the flush_rewrite_rules doc block to use "option" instead of "transient"

[david.binda]

In r12061 the temporary usage of transient for storing rewrite rules ( introduced in r10512 ) has been reverted back to option, but the documentation still says:

@param bool $hard Whether to update .htaccess (hard flush) or just update
rewrite_rules transient (soft flush). Default is true (hard).

It should be updated to rewrite_rules option to reflect the method WordPress is using for storing the rewrite rules.

[SergeyBiryukov]

In 47468:

Docs: Correct the description of $hard parameter in flush_rewrite_rules().

Props david.binda.
Fixes #49660.