Expand description of each component (#335)

## Description

<!-- Provide a brief description of the PR's purpose here. -->

Add more details to the description of each pyribs component.

## TODO

<!-- Notable points that this PR has either accomplished or will
accomplish. -->

## Questions

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## Status

- [x] I have read the guidelines in
[CONTRIBUTING.md](https://github.com/icaros-usc/pyribs/blob/master/CONTRIBUTING.md)
- [x] I have formatted my code using `yapf`
- [x] I have tested my code by running `pytest`
- [x] I have linted my code with `pylint`
- [ ] ~I have added a one-line description of my change to the changelog
in `HISTORY.md`~
- [x] This PR is ready to go

ribs/archives/__init__.py

@@ -1,4 +1,14 @@
-"""Archives store solutions found by a QD algorithm.
+"""Archive implementations and associated utilities.
+
+The archive is a data structure which stores solutions generated by the QD
+algorithm, along with any information relevant to solutions, such as objective
+and measure values. Archives in pyribs can also store metadata (i.e., arbitrary
+objects) alongside each solution.
+
+The archives in this subpackage are arranged in a one-layer hierarchy, with all
+archives inheriting from :class:`~ribs.archives.ArchiveBase`. This subpackage
+also contains several utilities associated with the archives, such as
+:class:`~ribs.archives.Elite` and :class:`~ribs.archives.ArchiveDataFrame`.
 
 .. autosummary::
     :toctree:

ribs/emitters/__init__.py

@@ -1,8 +1,10 @@
-"""Emitters output new candidate solutions in QD algorithms.
+"""Emitters generate new candidate solutions in QD algorithms.
 
-All emitters should inherit from :class:`EmitterBase`, except for emitters
-designed for differentiable quality diversity (DQD), which should instead
-inherit from :class:`DQDEmitterBase`.
+More formally, emitters are algorithms that generate solutions and adapt to
+objective, measure, and archive insertion feedback.
+
+The emitters in this module follow a one-layer hierarchy, with all emitters
+inheriting from :class:`~ribs.emitters.EmitterBase`.
 
 .. note::
     Emitters provided here take on the data type of the archive passed to their
@@ -23,7 +25,7 @@
 from ribs.emitters._evolution_strategy_emitter import EvolutionStrategyEmitter
 from ribs.emitters._gaussian_emitter import GaussianEmitter
 from ribs.emitters._gradient_arborescence_emitter import \
-  GradientArborescenceEmitter
+    GradientArborescenceEmitter
 from ribs.emitters._iso_line_emitter import IsoLineEmitter
 
 __all__ = [

ribs/schedulers/__init__.py

@@ -1,13 +1,21 @@
 """Schedulers link the entire QD algorithm together.
 
+Specifically, the scheduler performs two roles. First, the scheduler facilitates
+the interaction between the archive and the population of emitters. The
+scheduler adds solutions generated by emitters to the archive and passes the
+results of evaluation and archive insertion to the emitters. Second, schedulers
+select which emitters generate new solutions on each iteration of the algorithm.
+Schedulers make decisions on active emitters based on how well each emitter
+performs in previous iterations.
+
 .. autosummary::
     :toctree:
 
     ribs.schedulers.Scheduler
     ribs.schedulers.BanditScheduler
 """
-from ribs.schedulers._scheduler import Scheduler
 from ribs.schedulers._bandit_scheduler import BanditScheduler
+from ribs.schedulers._scheduler import Scheduler
 
 __all__ = [
     "Scheduler",