Random Docs Updates

[thegreatco]

• The DefaultInterface is mentioned in the Develop Cellular Apps page, but not mentioned on the OS & Device Configuration page.
• It is unclear what the proper mapping from Meadow -> MCU pins is, in the Develop Cellular Apps docs, it mentions a default value for the EnablePin of A3, but that does not appear on the Meadow -> MCU pin mapping tables in the Input/Output (I/O) page. There is also no link from the Develop Cellular Apps page to this table.
• It appears the Meadow -> MCU mapping table prefixes pin names with P, but the ReservedPins and EnablePin expect the pin name without the P prefix. Not sure if this is "wrong" in the table or in the cellular docs.
• Clarify that Resolver.Services treats everything as a singleton and use of disposed objects is possible if not careful.

[patridge]

The last Resolver.Services item should be tackled by the effort for #390.