Identify and fix references to non-existent functions in documentation

In rare occasions, there are references in the documentation to API names that do not exist.  For example, in the custom data source page, if you search for "WT_SESSION::msg_printf" and "WT_EXTENSION_API::config", you'll see that these strings exist, but there is no link, since there are no functions by those names.

Ideally, we'd not only fix these, but put into place a way to identify such broken references.  Here's a loose way to narrow the search:

$ (cd dist; bash s_docs)

$ (cd docs/; grep '[^>]WT_[A-Z_]*::' *.html | grep -v memname

This is nowhere close to perfect - it generates lots of false positives. For example, in upgrading.html it discusses that WT_SESSION::msg_printf has been removed from the API, so of course there wouldn't be a link.

If we can't enforce checking for broken references, we should at least scan and fix any we see using the method above or some other reliable method.