c-user: Add scheduler glossary terms

Add glossary terms which characterize the relationship of a scheduler to
a task.

c-user/glossary.rst

@@ -240,6 +240,10 @@ Glossary
         This term is an acronym for
         `Executable and Linkable Format <https://en.wikipedia.org/wiki/Executable_and_Linkable_Format>`_.
 
+    eligible scheduler
+        An eligible scheduler of a :term:`task` is a :term:`scheduler` which can be
+        used by the task to allocate a processor for the task.
+
     embedded
         An application that is delivered as a hidden part of a larger system.
         For example, the software in a fuel-injection control system is an
@@ -336,9 +340,20 @@ Glossary
         dispatch is marked as necessary, then the next thread dispatch will make
         the heir task the executing task.
 
+    helping scheduler
+        A helping scheduler of a :term:`task` is a :term:`scheduler` which is a
+        :term:`eligible scheduler` and which is not the :term:`home scheduler` of
+        the task.
+
     heterogeneous
         A multiprocessor computer system composed of dissimilar processors.
 
+    home scheduler
+        The home scheduler of a :term:`task` is a :term:`scheduler` which is a
+        :term:`eligible scheduler` and which is assigned to the task during the
+        initialization or explicitly via a directive call such as
+        :c:func:`rtems_task_set_scheduler`.
+
     homogeneous
         A multiprocessor computer system composed of a single type of processor.
 
@@ -352,6 +367,10 @@ Glossary
         A special low priority task which assumes control of the CPU when no
         other task is able to execute.
 
+    ineligible scheduler
+        An ineligible scheduler of a :term:`task` is a :term:`scheduler` which is
+        not an :term:`eligible scheduler`.
+
     interface
         A specification of the methodology used to connect multiple independent
         subsystems.
@@ -724,6 +743,11 @@ Glossary
         priority number and assign the tasks with the lowest priority number to
         one processor of the set of processors owned by a scheduler instance.
 
+        A scheduler is either an :term:`eligible scheduler` or a
+        :term:`ineligible scheduler` for a task.  An :term:`eligible scheduler` is
+        either the :term:`home scheduler` or a :term:`helping scheduler` for a
+        task.
+
     scheduler instance
         A scheduler instance is a scheduling algorithm with a corresponding
         context to store its internal state.  Each processor in the system is