Support document for __LINE__, __FILE__, __END__

[zzak]

This is from Bug #8157.

[drbrain]

Looking at this again, it is rather difficult to fix. These are all keywords, not methods, so there's nothing to link to. I'm unsure how to add them as a link target, but I'll leave this open in case I get an idea.

You can wrap them like <code>__FILE__</code>, but not +__FILE__+ (adding the latter seems to much of a special case to add).