docs/configuration: fix/update abstract_{chars_count,delimiter}

The description of abstract_delimiter was obviously wrong, copy-pasted
from abstract_chars_count. Fortunately, even if someone didn't notice
this, setting it to a numeric value erred out, therefore nobody can have
such a working configuration and there's no need to complicate things
trying to accept numbers. The best we can do is document the present
behaviour.

In order to make the interaction of these two settings clear, describe
the full mechanism at once and refer to it from the other place.

Author: tiosgz

docs/configuration.md

@@ -246,7 +246,8 @@ Used, in combination with `abstract_delimiter`, to determine each [item descript
 - Otherwise, the plugin first tries to retrieve the value of the keyword `description` from the [page metadata].
 - If that fails and `abstract_delimiter` is found in the page, the article content up to (but not including) the delimiter is used.
 - If the above has failed, then the plugin retrieves the first number of characters of the page content defined by this setting. Retrieved content is the raw markdown converted roughly into HTML.
-- Be careful: if set to `0` and there is no description, the feed's compliance is broken (an item must have a description).
+
+Be careful: if set to `0` and there is no description, the feed's compliance is broken (an item must have a description).
 
 `abstract_chars_count`: number of characters to use as item description.
 
@@ -256,9 +257,9 @@ Default: `150`
 
 ### `abstract_delimiter`: abstract delimiter
 
-Please see `abstract_chars_count` for how this setting is used.
+Please see `abstract_chars_count` for how this setting is used. A value of `""` (the empty string) disables this step.
 
-`abstract_delimiter`: string to mark .
+`abstract_delimiter`: string to mark where the description ends.
 
 Default: `<!-- more -->`