Document how --docSeeSrcUrl works

[dom96]

https://irclogs.nim-lang.org/29-01-2018.html#13:01:41

[timotheecour]

as mentioned here #6071 --docSeeSrcUrl doesn't seem to work

[dom96]

Duplicate of #6071.

@timotheecour you don't need to comment on both issues, your link from the other issue is reflected here and so this is becoming a bit too spammy.