[Issue 18400] New: Add an man page for ddemangle

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18400

          Issue ID: 18400
           Summary: Add an man page for ddemangle
           Product: D
           Version: D2
          Hardware: All
                OS: Linux
            Status: NEW
          Keywords: bootcamp
          Severity: enhancement
          Priority: P1
         Component: tools
          Assignee: nobody at puremagic.com
          Reporter: greensunny12 at gmail.com

See also: https://github.com/dlang/dmd/tree/master/docs and
https://github.com/dlang/tools/blob/master/man/man1/rdmd.1

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