Add more translator comments for placeholders

[nbachiyski]

There are many strings, in which there is no easy way for translators to know what the placeholders mean.

Here is a patch, which adds some comments for them.

[nbachiyski]

Added value: the patch also makes use of the separator in feed links.

[azaozz]

(In [11506]) Add more translator comments for placeholders, props nbachiyski, fixes #10002