documentation corrections

[nedbat]

• added string concatenation
• added ? as an alias for PRINT
• corrected the name of STR$
• fixed the values returned by = and <>
• corrected the description of > and >=
• fixed the mailto link
• other minor grammar fixes

Closes issues #5 and #7

BTW, I'm nearly done with an implementation in Python: https://github.com/nedbat/acidica. Thanks for this project, it's been a lot of fun!