add fzf's docs

src/algos/fzf/mod.rs

@@ -1,4 +1,47 @@
-//! TODO: docs
+//! Metrics implementing [fzf]'s fuzzy search algorithm.
+//!
+//! This module contains two metrics -- [`FzfV1`] and [`FzfV2`] -- both of
+//! which were ported directly from the original fzf project.
+//!
+//! ## `FzfV1` and `FzfV2`
+//!
+//! `FzfV1` is a simple metric that looks for the first "fuzzy" match of a
+//! query within a candidate. Its time complexity is `O(len(candidate))` for
+//! both matches and non-matches.
+//!
+//! `FzfV2` can often provide more accurate matches by finding the best
+//! possible position of a query within a candidate. This comes at the cost of
+//! a `O(len(query) * len(candidate))` time complexity for the distance
+//! calculation in the case of a match. Non-matches are still filtered out in
+//! `O(len(candidate))` time.
+//!
+//! For more information on the individual algorithms, refer to the
+//! documentation of each metric.
+//!
+//! ## Extended-search mode
+//!
+//! Both metrics fully support fzf's [extended-search mode][extended-search] by
+//! parsing the query with [`FzfParser::parse`].
+//!
+//! In extended-search mode, spaces in the query are treated as logical AND
+//! operators, while the pipe character `|` is treated as a logical OR. For
+//! example, the query `"foo bar | baz"` would only match candidates that
+//! contain `"foo"` and either `"bar"` or `"baz"`. It's also possible to query
+//! for candidates that either begin or end with a certain string, to negate a
+//! query, and more.
+//!
+//! To know more about extended-search mode's syntax you can
+//! look directly at [fzf's docs][extended-search] on it, or at the
+//! documentation of the [`FzfParser`].
+//!
+//! ## Conformance to fzf
+//!
+//! The behavior of both [`FzfV1`] and [`FzfV2`] is intended to always match
+//! that of fzf. Any discrepancy between our implementation and fzf's should be
+//! considered a bug.
+//!
+//! [fzf]: https://github.com/junegunn/fzf
+//! [extended-search]: https://github.com/junegunn/fzf#search-syntax
 
 mod common;
 mod distance;