stp: trim the comments, and put one back on the variable it describes

Review asked for this across the other commits too. Gone are the blocks
that restate what doc/stp.md already says, the ones that explain what an
embedded programmer already knows, and one that had gone stale inside this
very branch: the CLI summary above stp_parse still described "cost <0-255>
(x1000)" while the parser has taken the raw 0 to 200000000 for some time,
and it never learned about p2p or trk at all. A usage list next to the
parser is the kind of thing that rots first, so it is out rather than
updated.

The review flagged one comment saying a variable is in xdata because the
internal RAM overlay is full, on the grounds that it may stop being true.
Four more of the same kind were in these files and are out as well, one of
them pointing at a file that does not exist in this branch at all. The
declarations still say __xdata, which is the part a reader needs.

Also out: the note on why three helpers are __reentrant, which was really
a paragraph about two bytes of DSEG, and the measurement story behind the
tick divider, which belongs with the other timer numbers in doc/stp.md.

One comment was not stale but simply wrong. "max BPDUs per port per second"
sat on stp_failsafe_tripped, having slid down two lines when the two
failsafe variables were inserted above it. It describes stp_txhold and is
back there now.

Short factual labels stay: they sit next to the magic number they explain
and the codebase uses them throughout. The generated code is byte for byte
what it was before this commit, both banks and xdata unchanged.

httpd/page_impl.c

@@ -536,16 +536,10 @@ void send_lag(void)
 }
 
 
-/* STP status + configuration for the Spanning Tree page ("/stp.json").
- * Bridge config (prio index 0-15, hello/maxage/fwd seconds, rstp flag, tx
- * hold), elected root (priority byte + MAC), our path cost, root port, TC
- * counter, and per port: physical number, live ASIC state (2-bit MSTP field:
- * 0 Dis 1 Blk 2 Lrn 3 Fwd), an approximated role, and the per-port config
- * (enabled, edge admin/auto/oper, cost/1000, prio, guard, filter, tripped). */
+/* STP status and configuration for the Spanning Tree page ("/stp.json"). */
 __xdata uint8_t stp_we_root;
-__xdata uint8_t pi_i, pi_j, pi_j2;	/* shared loop iterators (DSEG relief) */
+__xdata uint8_t pi_i, pi_j, pi_j2;
 
-/* Parameter relays in xdata: keeps these helpers off the IRAM overlay */
 static __xdata uint32_t pi_u32;
 static __xdata uint8_t pi_prio, pi_ext;
 static __xdata uint8_t * __xdata pi_mac;