Client lua api documentation: .md -> .txt

Client lua api documentation: .md -> .txt

For consistency, and for much improved access on Github.

On Github:
.md disables being able to link to a line, needed daily when
quickly referring people to particular lines.
Search returns results in .md files, but you cannot then click
the line number to go to that line in the document, making searching
difficult.
Line numbers are not displayed for .md documents.