Ensure inline code is markdown-escaped as such, HTML tags are removed from summaries, and that code snippets in descriptions are properly indented.

Affects DocBlocks for the following core elements:
* Remove HTML tag from parameter description in `comment_form()`
* Remove HTML tag from a summary for the `comment_form_top` hook
* Markdown-indent a code snippet in the description for `get_linkobjectsbyname()`
* Markdown-indent a code snippet and format an unordered list in the description for `get_linkobjects()`
* Backtick-escape some inline code in the description for `clean_pre()`
* Remove HTML tag from the summary for the `rss_tag_pre` hook
* Various formatting fixes in the descriptions for `get_filesystem_method()` and `request_filesystem_credentials()`

Props rarst for the initial patch.
See #30473.

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