doc: add manpage for priority factors

Problem: There is no documentation for configuring the different
factors used in calculating a job's priority.

Add a new manpage to the doc/ folder that outlines the configuration
process for the various factors used in the priority plugin.

doc/man5/flux-config-accounting.rst

@@ -0,0 +1,45 @@
+=========================
+flux-config-accounting(5)
+=========================
+
+
+DESCRIPTION
+===========
+
+The flux-accounting priority plugin can be configured to assign different
+weights to the different factors used when calculating a job's priority.
+Assigning a higher weight to a factor will result in it having more
+influence in the calculated priority for a job. By default, the factors in
+the priority plugin have the following weights:
+
++-------------+--------+
+| factor      | weight |
++=============+========+
+| fairshare   | 100000 |
++-------------+--------+
+| queue       | 10000  |
++-------------+--------+
+
+The ``accounting.factor-weights`` sub-table may contain the following keys:
+
+
+KEYS
+====
+
+fairshare
+    Integer value that represents the weight to be associated with an
+    association's fairshare value.
+
+queue
+   Integer value that represents the weight associated with submitting a job
+   to a certain queue.
+
+
+EXAMPLE
+=======
+
+::
+
+   [accounting.factor-weights]
+   fairshare = 10000
+   queue = 1000