Two media gallery string changes, for clarity:

 * 'Random' to 'Random Order', to be clear what it controls.
 * 'Describe this image...' to 'Caption this image...', to be clear it links to the Caption field and not the revived Description field, see [23083].

props johnbillion, pavelevap.
fixes #22821.



git-svn-id: https://core.svn.wordpress.org/trunk@23135 1a063a9b-81f0-0310-95a4-ce76da25c4cd