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

"Running under upaas" in README.md listed PIXA_CACHE_MAX_BYTES as
optional. Under upaas the volume is a directory on the host's
filesystem, and the default limit is 75% of the sum of that
filesystem's free space and what the cache holds, at least 500 MiB, so
one pixa can take three quarters or more of the space every app has
left, and all of it when that is 500 MiB or less. The section now tells
the operator to set the variable and says what the default does there.
The default is unchanged.

Model: opus-5-5

README.md

@@ -123,8 +123,12 @@ 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. The default is 75% of the sum of that
+      filesystem's free space and what the cache holds, and at least 500 MiB, so
+      the cache can grow to three quarters or more of the space they all have
+      left, and to all of it when that is 500 MiB or less.
     - 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