diff --git a/_pytest/hookspec.py b/_pytest/hookspec.py
index ea1f0443d..552a06575 100644
--- a/_pytest/hookspec.py
+++ b/_pytest/hookspec.py
@@ -263,7 +263,14 @@ def pytest_assertrepr_compare(config, op, left, right):
 # -------------------------------------------------------------------------
 
 def pytest_report_header(config, startdir):
-    """ return a string to be displayed as header info for terminal reporting."""
+    """ return a string to be displayed as header info for terminal reporting.
+
+    .. note::
+
+        This function should be implemented only in plugins or ``conftest.py``
+        files situated at the tests root directory due to how pytest
+        :ref:`discovers plugins during startup <pluginorder>`.
+    """
 
 @hookspec(firstresult=True)
 def pytest_report_teststatus(report):