Update reference to renamed folder: `doc` → `docsrc`

See e2ec85bdae4227b32437456b2f518bdc2ada226c.
Add missing @format to external Xrefs
Update Xref text from current IETF BCP 47 doc title

"Tags for Identifying Languages"
Change @type to @format for external Xrefs
Merge pull request #1748 from infotexture/feature/docfixes

Minor updates to documentation source files