Don't use a variable variable in `wp_widget_rss_form()`. Sidenote: the logic to show hidden fields is bizarre - would result in duplicate fields.

See #27881.

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git-svn-id: http://core.svn.wordpress.org/trunk@28548 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Improve inline documention for `set_transient()` and `set_site_transient()` to specify the 45- and 40-character limits for their respective transient name values.

Props edwin-at-studiojoyo.com for the original patch.
See #15058, #13310. Fixes #28467.

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git-svn-id: http://core.svn.wordpress.org/trunk@28549 1a063a9b-81f0-0310-95a4-ce76da25c4cd