Docs fixes (#448)

## Description

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

More fixes to documentation.

## TODO

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

- [x] Mention single-field usage in error msg for as_pandas
- [x] Fix return_type docs for archive data method and ArrayStore
retrieve method

## 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`
- [N/A] 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/_archive_base.py

@@ -643,15 +643,18 @@ def data(self, fields=None, return_type="dict"):
         """Retrieves data for all elites in the archive.
 
         Args:
-            fields (array-like of str): List of fields to include. By default,
-                all fields will be included (see :attr:`field_list`), with an
-                additional "index" as the last field ("index" can also be placed
-                anywhere in this list).
-            return_type (str): Type of data to return. See below.
+            fields (str or array-like of str): List of fields to include. By
+                default, all fields will be included, with an additional "index"
+                as the last field ("index" can also be placed anywhere in this
+                list). This can also be a single str indicating a field name.
+            return_type (str): Type of data to return. See below. Ignored if
+                ``fields`` is a str.
 
         Returns:
-            The data at the given indices. This can take the following forms,
-            depending on the ``return_type`` argument:
+            The data for all entries in the archive. If ``fields`` was a single
+            str, this will just be an array holding data for the given field.
+            Otherwise, this data can take the following forms, depending on the
+            ``return_type`` argument:
 
             - ``return_type="dict"``: Dict mapping from the field name to the
               field data at the given indices. An example is::
@@ -675,7 +678,8 @@ def data(self, fields=None, return_type="dict"):
               ``(objective_arr, measures_arr)``. In this case, the results
               from ``retrieve`` could be unpacked as::
 
-                  objective, measures = archive.data(["objective", "measures"])
+                  objective, measures = archive.data(["objective", "measures"],
+                                                     return_type="tuple")
 
               Unlike with the ``dict`` return type, duplicate fields will show
               up as duplicate entries in the tuple, e.g.,
@@ -725,11 +729,9 @@ def as_pandas(self, include_solutions=True, include_metadata=False):
         # pylint: disable = unused-argument
         raise RuntimeError(
             "as_pandas has been deprecated. Please use "
-            "archive.data(..., return_type='pandas') instead. For more "
-            "info, please see the archive data tutorial: "
-            # pylint: disable = line-too-long
-            "https://docs.pyribs.org/en/stable/tutorials/features/archive_data.html"
-        )
+            "archive.data(..., return_type='pandas') instead, or consider "
+            "retrieving individual fields, e.g., "
+            "objective = archive.data('objective')")
 
     def cqd_score(self,
                   iterations,

ribs/archives/_array_store.py

@@ -257,7 +257,10 @@ def retrieve(self, indices, fields=None, return_type="dict"):
                 ``(objective_arr, measures_arr)``. In this case, the results
                 from ``retrieve`` could be unpacked as::
 
-                    occupied, (objective, measures) = store.retrieve(...)
+                    occupied, (objective, measures) = store.retrieve(
+                        ...,
+                        return_type="tuple",
+                    )
 
                 Unlike with the ``dict`` return type, duplicate fields will show
                 up as duplicate entries in the tuple, e.g.,