hookspec: improve collection phase documentation a bit

Make it a bit more accurate and use the same format that
pytest_runtest_protocol uses.

src/_pytest/hookspec.py

@@ -205,20 +205,32 @@ def pytest_load_initial_conftests(
 
 @hookspec(firstresult=True)
 def pytest_collection(session: "Session") -> Optional[object]:
-    """Perform the collection protocol for the given session.
+    """Perform the collection phase for the given session.
 
     Stops at first non-None result, see :ref:`firstresult`.
     The return value is not used, but only stops further processing.
 
-    The hook is meant to set `session.items` to a sequence of items at least,
+    The default collection phase is this (see individual hooks for full details):
-    but normally should follow this procedure:
 
-      1. Call the pytest_collectstart hook.
+    1. Starting from ``session`` as the initial collector:
-      2. Call the pytest_collectreport hook.
+
-      3. Call the pytest_collection_modifyitems hook.
+      1. ``pytest_collectstart(collector)``
-      4. Call the pytest_collection_finish hook.
+      2. ``report = pytest_make_collect_report(collector)``
-      5. Set session.testscollected to the amount of collect items.
+      3. ``pytest_exception_interact(collector, call, report)`` if an interactive exception occurred
-      6. Set `session.items` to a list of items.
+      4. For each collected node:
+
+        1. If an item, ``pytest_itemcollected(item)``
+        2. If a collector, recurse into it.
+
+      5. ``pytest_collectreport(report)``
+
+    2. ``pytest_collection_modifyitems(session, config, items)``
+
+      1. ``pytest_deselected(items)`` for any deselected items (may be called multiple times)
+
+    3. ``pytest_collection_finish(session)``
+    4. Set ``session.items`` to the list of collected items
+    5. Set ``session.testscollected`` to the number of collected items
 
     You can implement this hook to only perform some action before collection,
     for example the terminal plugin uses it to start displaying the collection
@@ -286,7 +298,10 @@ def pytest_collectreport(report: "CollectReport") -> None:
 
 
 def pytest_deselected(items: Sequence["Item"]) -> None:
-    """Called for deselected test items, e.g. by keyword."""
+    """Called for deselected test items, e.g. by keyword.
+
+    May be called multiple times.
+    """
 
 
 @hookspec(firstresult=True)

src/_pytest/main.py

@@ -538,6 +538,20 @@ class Session(nodes.FSCollector):
     def perform_collect(  # noqa: F811
         self, args: Optional[Sequence[str]] = None, genitems: bool = True
     ) -> Sequence[Union[nodes.Item, nodes.Collector]]:
+        """Perform the collection phase for this session.
+
+        This is called by the default
+        :func:`pytest_collection <_pytest.hookspec.pytest_collection>` hook
+        implementation; see the documentation of this hook for more details.
+        For testing purposes, it may also be called directly on a fresh
+        ``Session``.
+
+        This function normally recursively expands any collectors collected
+        from the session to their items, and only items are returned. For
+        testing purposes, this may be suppressed by passing ``genitems=False``,
+        in which case the return value contains these collectors unexpanded,
+        and ``session.items`` is empty.
+        """
         hook = self.config.hook
         try:
             items = self._perform_collect(args, genitems)