Nim: Document the compiler configuration file format

[Varriount]

Or at least make the existing documentation more apparent.
Aside from a basic understanding that the configuration file format is somewhat like the ini file format, not much else is stated. At the very least, the conditional variables that the compiler uses when parsing the files should be documented.

[Varriount]

Notes:

• Config parser uses defines found in condsyms.nim with '@' expressions
• successive definitions of the same identifier are appended, not overwritten

[filcuc]

i think that this should have a higher priority