fix: cache button renders on single page; cache_button edit swallows deleted/expired message (v0.1.7)

update_buttons gated the cache button behind the nav row, so a single-page
paginator with a non-empty cache never showed it despite the README promising
otherwise; the cache button now renders whenever self.cache is truthy,
independent of nav, and start() keeps the view alive for that case too.

cache_button's post-sleep message.edit was unguarded, so a message deleted or
expired during cache_sleep raised out of the callback; it now mirrors
on_timeout's NotFound/Forbidden swallow and HTTPException log.

also compresses the module and method docstrings (no behavior change).

Signed-off-by: disqualifier <dev@disqualifier.me>

README.md

@@ -9,18 +9,18 @@ buttons) behind previous / jump / next navigation, with an optional cache button
 `requirements.txt`:
 
 ```
-dpy_paginator @ git+ssh://git@git.rethinkstudios.io/rethink-public/dpy_paginator.git@v0.1.6
+dpy_paginator @ git+ssh://git@git.rethinkstudios.io/rethink-public/dpy_paginator.git@v0.1.7
 ```
 
 Direct:
 
 ```bash
-pip install "dpy_paginator @ git+ssh://git@git.rethinkstudios.io/rethink-public/dpy_paginator.git@v0.1.6"
+pip install "dpy_paginator @ git+ssh://git@git.rethinkstudios.io/rethink-public/dpy_paginator.git@v0.1.7"
 ```
 
 Requires `discord.py` (pulled transitively).
 
-Drop the `@v0.1.6` suffix from the line above to install the latest unpinned.
+Drop the `@v0.1.7` suffix from the line above to install the latest unpinned.
 
 ## Basic usage
 
@@ -134,6 +134,10 @@ Omit `cache` or pass `None`/`[]` and the button never appears. When set, `cache`
 must have one entry per rendered page — a shorter `cache` raises `ValueError` at
 construction rather than failing with an `IndexError` mid-navigation.
 
+The cache button renders whenever `cache` is truthy, even on a single page (no
+navigation row) — it doesn't depend on there being more than one page to flip
+through.
+
 ## Constructor options
 
 - `pages` — sequence of page content