doc: expand input documentation for flux job taskmap

Problem: The documentation of `flux job taskmap` in flux-job(1)
does not note that the command can also take input in raw format.

Add this input type to the list described in the documentation of
the `--to` option.

Also correct one typo on the following line.

Author: grondo

doc/man1/flux-job.rst

@@ -349,9 +349,12 @@ support task mapping formats:
 .. option:: --to=raw|pmi|multiline
 
    Convert the taskmap to *raw* or *pmi* formats (described in RFC 34), or
-   *multiline* which prints the node ID of each task, one per line.
+   *multiline* which prints the node ID of each task, one per line. The
+   default behavior is to print the RFC 34 taskmap. This option can be useful
+   to convert between mapping forms, since :program:`flux job taskmap` can
+   take a raw, pmi, or RFC 34 task map on the command line.
 
-One one of the above options may be used per call.
+Only one of the above options may be used per call.
 
 timeleft
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