doc: Removed all definition lists as Sundown doesn't support them.

Closes #13561. Please revert this commit when Sundown (or any
replacement) gets a support for definition lists in the future.
auto merge of #13563 : lifthrasiir/rust/refman-dl, r=alexcrichton

Closes #13561. All definition lists have been converted to unordered lists. This is a temporary measure; please revert this when Sundown (or any replacement) gets a support for definition lists in the future.