Improve documentation slightly

All those comments in pdb_dpm.h aren't needed anymore.  They were just
to help me organize my thoughts on how the DPM should work.

lib/include/pdb.h

  * be statically allocated!
  */
 struct pdb_config {
+    /* User-initialized fields */
     /* Configuration information for the FUSB302B* chip */
     struct pdb_fusb_config fusb;
     /* DPM callbacks */
     struct pdb_dpm_callbacks dpm;
     /* Pointer to port-specific DPM data */
     void *dpm_data;
+
+    /* Automatically initialized fields */
     /* Policy Engine thread and related variables */
     struct pdb_pe pe;
     /* Protocol layer threads and related variables */

lib/include/pdb_dpm.h

  * PD Buddy firmware library device policy manager callback structure
  *
  * Optional functions may be set to NULL if the associated functionality is not
- * required.
+ * required. (TODO)
  */
 struct pdb_dpm_callbacks {
     pdb_dpm_eval_cap_func evaluate_capability;
     pdb_dpm_giveback_func giveback_enabled;
     pdb_dpm_tcc_func evaluate_typec_current; /* Optional */
     pdb_dpm_func pd_start; /* Optional */
-    /* dpm_sink_standby is called in PE_SNK_Select_Capability to ensure power
-     * consumption is less than 2.5 W.
-     *   - dpm_transition_standby()
-     */
-    /* dpm_output_set is called in five places:
-     *   - PS_RDY received → transition sink power supply to our request
-     *     - This could mean what we wanted, or what we had to settle for, so
-     *       we have to remember.
-     *     - dpm_transition_requested()
-     *   - Protocol error in PE_SNK_Transition_Sink
-     *     - Unnecessary?  If we really want to be defensive here, just go to
-     *       default output, right?  That's what we'll do shortly afterwards
-     *       anyway.
-     *     - dpm_transition_default()
-     *   - GotoMin received and giveback → transition sink to min power
-     *     - dpm_transition_min()
-     *   - Transition sink power supply to match Type-C Current advertisement
-     *     - dpm_transition_typec() (if I don't like it I can change it)
-     *   - Part of dpm_output_default
-     *     - Obsolete (see below)
-     */
-    /* dpm_output_default is only called in PE_SNK_Transition_to_Default.
-     * Should probably be part of new improved replacement for dpm_output_set.
-     *   - dpm_transition_default()
-     */
     pdb_dpm_func transition_default;
     pdb_dpm_func transition_min; /* Optional if no GiveBack */
     pdb_dpm_func transition_standby;