feat[docs]: Add Infinity, NaN, and boolean literals.

docs/language.md

@@ -284,10 +284,17 @@ float6 = -2E+4 # same as -20000.0
 float7 = 5_000.000_003 # same as 5000.000003
 ```
 
+Use `@inf` and `@nan` to express positive infinity and "Not a number."
+
+```toml
+float8 = @inf # Positive Infinity
+float9 = -@inf # Negative Infinity
+float10 = @nan # Not a number.
+```
+
 #### Boolean
-Boolean is a value, either "true" or "false". Booleans has no literal
-notation, but can be accessed as [master key](#master-key) constants: `@true`
-and `@false`.
+Boolean is a value, either "true" or "false". Use `@true` and `@false`
+to express them. (`true` and `false` are keys.)
 
 ```toml
 bool1 = @true