Hook "do_mu_upgrade" docblock in wrong line

[ChriCo]

Currently the hook do_mu_upgrade in wp-admin/admin.php L70 has a dockblock which is inside the if-statement and therefor not that easy to parse out for automatic documentation. Also this statement could be improved by adding some early return to the first if - which is already there as exit.

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Patch contains:

1. Replaced {{elseif}} by {{if}}
2. Moved {{apply_filters}} above the {{if}} to have documentation at the correct position

[SergeyBiryukov]

In 45643:

Docs: Move the do_mu_upgrade DocBlock out of the preceding if statement.

Props ChriCo, dkarfa.
Fixes #47710.