Update documentation

Author: gwillen

src/init.cpp

@@ -425,7 +425,7 @@ void SetupServerArgs(ArgsManager& argsman)
     hidden_args.emplace_back("-sysperms");
 #endif
     argsman.AddArg("-txindex", strprintf("Maintain a full transaction index, used by the getrawtransaction rpc call (default: %u)", DEFAULT_TXINDEX), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
-    argsman.AddArg("-trim_headers", strprintf("Trim old headers in memory, removing blocksigning and dynafed-related fields. Saves memory, but blocks us from serving blocks or headers to peers, and removes trimmed fields from some JSON RPC outputs. (default: false)"), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
+    argsman.AddArg("-trim_headers", strprintf("Trim old headers in memory (by default older than 2 epochs), removing blocksigning and dynafed-related fields. Saves memory, but blocks us from serving blocks or headers to peers, and removes trimmed fields from some JSON RPC outputs. (default: false)"), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-blockfilterindex=<type>",
                  strprintf("Maintain an index of compact filters by block (default: %s, values: %s).", DEFAULT_BLOCKFILTERINDEX, ListBlockFilterTypes()) +
                  " If <type> is not supplied or if <type> = 1, indexes for all known types are enabled.",

src/txdb.cpp

@@ -307,6 +307,9 @@ bool CBlockTreeDB::WritePAKList(const std::vector<std::vector<unsigned char> >&
         return Write(std::make_pair(DB_PAK, uint256S("1")), offline_list) && Write(std::make_pair(DB_PAK, uint256S("2")), online_list) && Write(std::make_pair(DB_PAK, uint256S("3")), reject);
 }
 
+/** Note that we only get a conservative (lower) estimate of the max header height here,
+ * obtained by sampling the first 10,000 headers on disk (which are in random order) and
+ * taking the highest block we see. */
 bool CBlockTreeDB::WalkBlockIndexGutsForMaxHeight(int* nHeight) {
     std::unique_ptr<CDBIterator> pcursor(NewIterator());
     *nHeight = 0;