Add XML documentation for settings which have no documentation

[angelozerr]

Once #326 will be merged, it should be nice to improve documentation by collecting settings (ex : xml.validation.resolveExternalEntities) which are not linked to documentation

[datho7561]

Undocumented settings (no link to internal docs) as of commit c90c732. I only included ones that need a longer description.

• XML code lens enabled
• XML completion auto close tags don't think this is worth another entry
• XML file associations
• XML formatting Xsi Schema Location split
• XML java home (maybe we don't need much)
• XML preferences show schema documentation type
• XML validation disallow doc type decl
• XML validation resolve external entities

Using this to keep track of progress documenting the settings as well.
Updating the wiki with the changes first, then adding a link here. Once the documentation is in my internal documentation PR draft, I will check off the item.

[angelozerr]

For XML code lens,

• I will create a CodeLens.md for that because you have other codelens kind (ex : xs:element)
• add a section about Code lens for xs:element -> see https://developers.redhat.com/blog/2019/07/24/whats-new-in-the-visual-studio-code-xml-extension/
• add screenshot (or better a demo) for codelens to show references are clickable.