tracing: misc follow-ups to 22902

- mention 'Lost X events' workaround
- clarify flush tracepoint docs
- fix typo in tracepoint context
- clarify flush for prune
    The documentation and examples for the `fFlushForPrune` argument
    of the utxocache flush tracepoint weren't clear without looking
    at the code.

    See these comments: https://github.com/bitcoin/bitcoin/pull/22902#issuecomment-987094612

- doc: note that there can be temporary UTXO caches
    Bitcoin Core uses temporary clones of it's _main_ UTXO cache in some
    places. The utxocache:add and :spent tracepoints are triggered when
    temporary caches are changed too. This is documented.

contrib/tracing/log_utxos.bt

@@ -7,7 +7,7 @@
   bpftrace contrib/tracing/log_utxos.bt
 
   This script requires a 'bitcoind' binary compiled with eBPF support and the
-  'utxochache' tracepoints. By default, it's assumed that 'bitcoind' is
+  'utxocache' tracepoints. By default, it's assumed that 'bitcoind' is
   located in './src/bitcoind'. This can be modified in the script below.
 
   NOTE: requires bpftrace v0.12.0 or above.