Name the refusal rule, and refuse a period the encoder cannot spell (#99)

* Refuse a relative validity period the SMPP time format cannot hold

* Export unencodableText(), so a caller can word a refusal as the library does

* Name the refusal rule in the README, and correct what it claims

* Word the period refusal by what the encoder spells, and keep unencodableText() internal

* Correct the decision bullet premise about what the relative format holds

* Close the stability nits on the time guard and its tests

README.md

@@ -110,6 +110,11 @@ await session.sendSms({
 }, { signal });                       // optional per-call AbortSignal
 ```
 
+One rule decides which choices of yours this library second-guesses, here and on `session.send()`:
+it checks what it composes for you — an alphabet or a time you named, a string body under a
+`data_coding` you named — and passes on whatever you formed yourself, a `Buffer` body or a stamp you
+formatted.
+
 `sourceAddrTon` and `destinationAddrTon` default to 5 for an alphanumeric address and 1 for a
 numeric one; the NPI fields default to 0. Set them for an operator that requires something else.
 
@@ -125,16 +130,22 @@ segment goes out.
 `encoding` names the alphabet, and is `ASCII` (GSM 03.38's own 7-bit table), `LATIN1` or `UCS2`;
 anything else is refused by name rather than guessed at. Leave it out and a message that fits GSM
 7-bit goes as `ASCII` and everything else as `UCS2`; `LATIN1` is only ever used when you name it.
+`consts.ENCODING` is the low-level surface's `data_coding` table rather than this option's list,
+`FLASH` and the alphabets no codec here implements included.
 
 An alphabet you name has to carry the message: `LATIN1` beside a character above U+00FF, or `ASCII`
 beside one GSM 03.38 has no code for, is refused before anything goes out, and the error names the
 character, its code point and where in the message it is. Leaving `encoding` out is never refused —
-what detection picks always fits. `LATIN1` still carries every octet, so an 8-bit binary body sent
-as `buffer.toString('latin1')` goes out unchanged.
+what detection picks always fits. `LATIN1` carries every octet, so an 8-bit body passed as
+`buffer.toString('latin1')` reaches the SMSC byte for byte — but under `data_coding` 0x03, which
+declares Latin-1 text rather than binary. To declare 8-bit binary, hand `session.send()` a `Buffer`
+body and the `data_coding` you want; see [Working with PDUs directly](#working-with-pdus-directly).
 
 `scheduleDeliveryTime` and `validityPeriod` take a `Date`, a number of seconds, or a stamp you
 formatted yourself. One that names no time — an invalid `Date`, `NaN`, `Infinity` — is refused
-before anything goes out.
+before anything goes out, and so is a negative number of seconds or one past 99 days 23:59:59: a
+period counted in seconds is spelled in days and below, since no fixed number of them is a month or
+a year. Name an instant further out as a `Date`, which goes out absolute.
 
 `flash` asks for GSM 03.38 message class 0, the class a handset shows on arrival instead of storing.
 It travels in `data_coding` beside the alphabet, so a flash UCS2 message stays UCS2. Pairing it with
@@ -431,7 +442,9 @@ A refused inbound `deliver_sm` is lost traffic: a message or a receipt that neve
 `dlr`, and this event is where that loss shows up. A refused *response* is reported twice where a
 call is still waiting on it, once as the `err` that `sendSms()` or `send()` returns and once here.
 That is deliberate: the call answers what became of that one send, and the event is what shows a peer
-answering unreadably at all.
+answering unreadably at all. A `PduRefusedError` is always a PDU that arrived, so that is the only
+way one reaches a send's `err`: what this library refuses to build or send — an alphabet, a time, a
+body a `data_coding` cannot carry — is a plain `Error` in the result.
 
 ## Logging