Merge bitcoin#13676: Explain that mempool memory is added to -dbcache

7cb1a14 Explain that unused mempool memory is added to -dbcache (Sjors Provoost)

Pull request description:

  Since `-maxmempool` is 450 MB by default it's quite possible for a user to accidentally OOM a low
  memory device if they increase `-dbcache` beyond the default.

  <img width="563" alt="schermafbeelding 2018-09-06 om 17 02 40" src="https://user-images.githubusercontent.com/10217/45166219-c9c4f700-b1f6-11e8-9ee5-14b8b3a9830b.png">

Tree-SHA512: 44c7419d0b06c14aee5d2c02a41e5da488bcb40a5f65ba24554a45707b222f1e4b03d42486dfef9336d917ac2990eef2b1aec287a75b3ef1ccca0e88ac86a0c0

src/init.cpp

@@ -463,7 +463,7 @@ void SetupServerArgs()
     gArgs.AddArg("-conf=<file>", strprintf("Specify configuration file. Relative paths will be prefixed by datadir location. (default: %s)", BITCOIN_CONF_FILENAME), false, OptionsCategory::OPTIONS);
     gArgs.AddArg("-datadir=<dir>", "Specify data directory", false, OptionsCategory::OPTIONS);
     gArgs.AddArg("-dbbatchsize", strprintf("Maximum database write batch size in bytes (default: %u)", nDefaultDbBatchSize), true, OptionsCategory::OPTIONS);
-    gArgs.AddArg("-dbcache=<n>", strprintf("Set database cache size in MiB (%d to %d, default: %d)", nMinDbCache, nMaxDbCache, nDefaultDbCache), false, OptionsCategory::OPTIONS);
+    gArgs.AddArg("-dbcache=<n>", strprintf("Maximum database cache size <n> MiB (%d to %d, default: %d). In addition, unused mempool memory is shared for this cache (see -maxmempool).", nMinDbCache, nMaxDbCache, nDefaultDbCache), false, OptionsCategory::OPTIONS);
     gArgs.AddArg("-debuglogfile=<file>", strprintf("Specify location of debug log file. Relative paths will be prefixed by a net-specific datadir location. (-nodebuglogfile to disable; default: %s)", DEFAULT_DEBUGLOGFILE), false, OptionsCategory::OPTIONS);
     gArgs.AddArg("-includeconf=<file>", "Specify additional configuration file, relative to the -datadir path (only useable from configuration file, not command line)", false, OptionsCategory::OPTIONS);
     gArgs.AddArg("-loadblock=<file>", "Imports blocks from external blk000??.dat file on startup", false, OptionsCategory::OPTIONS);