Text.Lexer: add digit lexer and correct docs
Text.Lexer: add digit lexer and correct docs

`digit` recognises a single digit.

uuhan/Idris-dev,uuhan/Idris-dev,kojiromike/Idris-dev,uuhan/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev,uuhan/Idris-dev,uuhan/Idris-dev,kojiromike/Idris-dev,uuhan/Idris-dev,kojiromike/Idris-dev,uuhan/Idris-dev,kojiromike/Idris-dev