strip trailing whitespace in generated docstrings

[visr]

And also remove remove empty lines in Parameters or Returns lists. Though it could be that I don't understand the reason those need to be there. The markdown format doesn't require them.

• remove trailing whitespace for Clang.Paragraph
  see the effect in JuliaGeo/Proj.jl@04b9fb8
• remove empty lines in Parameters or Returns lists
  see the effect in JuliaGeo/Proj.jl@fc126ae
• strip trailing whitespace after parameter
  see the effect in JuliaGeo/Proj.jl@7830d54

[melonedo]

From Julia manual on markdown lists, the empty line is required for complex markdown elements. For example a nested list in parameter.

## Parameters
- `arg`: This arguments takes various forms:
    - form1
    - form2
- `arg2`: code block

    ```c
    int foo(void);
    ````

My experiments showed no difference though.

[Gnimuc]

That doc is probably outdated. This change might be related.

[melonedo]

So we can omit the empty line as long as we do not find broken format. I could not reproduce what formatting issue might be triggered by omitting that.