Properly interpret double slashes in documentation

[zatherz]

\\ escapes were previously not included in the regex (despite having a mapping).
Older versions of DCR returned \n both for new lines and literal \ and n characters, which caused DKit to misbehave. For example, the description of writeln looked like this:

Equivalent to `write(args, '
')`.  Calling `writeln` without
arguments is valid and just prints a newline to the standard
output.

This was fixed in some version of DCR (it now returns \\n when it's a literal \n and still just a \n when it's a line break), but without this change, DKit shows the description of writeln as:

Equivalent to `write(args, '\\n')`.  Calling `writeln` without
arguments is valid and just prints a newline to the standard
output.

With this tiny fix, it now shows up as:

Equivalent to `write(args, '\n')`.  Calling `writeln` without
arguments is valid and just prints a newline to the standard
output.

[yazd]

As this looks independent, I'll merge it on its own. Thanks.