Add a note to the user guide on limiting disk cache growth

docs/UserGuide.md

@@ -341,6 +341,7 @@ Termshark reads options from a TOML configuration file saved in `$XDG_CONFIG_HOM
 - `copy-command-timeout` (int) - how long termshark will wait (in seconds) for the copy command to complete before reporting an error.
 - `dark-mode` (bool) - if true, termshark will run in dark-mode.
 - `disable-shark-fin` (bool) - if true then it's safe to go back in the water.
+- `disk-cache-size-mb` (int) - how large termshark will allow `$XDG_CACHE_HOME/termshark/pcaps/` to grow; if the limit is exceeded, termshark will delete pcaps, oldest first. Set to -1 to disable (grow indefinitely).
 - `dumpcap` (string) - make termshark use this specific `dumpcap` (used when reading from an interface).
 - `packet-colors` (bool) - if true (or missing), termshark will colorize packets according to Wireshark's rules.
 - `pcap-bundle-size` - (int) - load tshark PDML this many packets at a time. Termshark will lazily load PDML because it's a slow process and uses a lot of RAM. For example, if `pcap-bundle-size`=1000, then on first loading a pcap, termshark will load PDML for packets 1-1000. If you scroll past packet 500, termshark will optimistically load PDML for packets 1001-2000. A higher value will make termshark load more packets at a time; a value of 0 means load the entire pcap's worth of PDML. Termshark stores the data compressed in RAM, but expect approximately 10MB per 1000 packets loaded. If you have the memory, can wait a minute or two for the entire pcap to load, and e.g. plan to use the packet list header to sort the packets in various ways, setting `pcap-bundle-size` to 0 will provide the best experience.