doc: document shell.post-init in flux-shell(1)

Problem: The new shell.post-init shell plugin callback topic is
not documented.

Add it to flux-shell(1).

Author: grondo

doc/man1/flux-shell.rst

@@ -71,6 +71,7 @@ execution of the job:
  * change working directory to the cwd of the job
  * enter a barrier to ensure shell initialization is complete on all shells
  * emit ``shell.init`` event to exec.eventlog
+ * call ``shell.post-init`` plugin callbacks
  * create all local tasks. For each task, the following procedure is used
 
    - call ``task.init`` plugin callback
@@ -142,6 +143,10 @@ topics:
   **jobspec** and **R** from the KVS, but before any tasks
   are started.
 
+**shell.post-init**
+  Called after the shell initialization barrier has completed, but
+  before starting any tasks.
+
 **task.init**
   Called for each task after the task info has been constructed
   but before the task is executed.