Tell upaas operators to set the cache size limit (closes #251)

"Running under upaas" listed PIXA_CACHE_MAX_BYTES as optional. Under
upaas the volume is a directory on the host's filesystem, so the
default, 75% of that filesystem's free space plus what the cache holds,
lets one pixa take three quarters of the space the host and every other
app have left. The section now says to set it and why. The default is
unchanged.

Model: opus-5-5

README.md

@@ -123,8 +123,11 @@ What the [upaas](https://git.eeqj.de/sneak/upaas) app for pixa needs:
       comma-separated; default none. Every image on a listed host is served to
       anyone without a signature, so list only hosts you control, where nobody
       else can put images.
-    - `PIXA_CACHE_MAX_BYTES`: disk cache limit in bytes; `0` disables it;
-      default 75% of (free space + what the cache holds)
+    - `PIXA_CACHE_MAX_BYTES`: disk cache limit in bytes; `0` disables it. Set
+      it: the volume is a directory on the host's filesystem, shared with the
+      host and every other app, and the default, 75% of the sum of that
+      filesystem's free space and what the cache holds, lets the cache grow to
+      three quarters of the space they all have left.
     - the rest are in the table under Configuration below
 - **Health check:** the image's `HEALTHCHECK` requests
   `/.well-known/healthcheck.json`. upaas reads the container's health 60 seconds