doc: Correct comment about which subsystem detects lagging clocks #36134

Turns out a completely fresh datadir means there is no chain state to load and hence no detection of a lagging clock occurs in that subsystem. Instead we do proceed into attempting to start a headers sync.

<details><summary>Diff to repro with fresh -datadir</summary>

--- a/src/init.cpp
+++ b/src/init.cpp
@@ -1499,6 +1499,8 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     const ArgsManager& args = *Assert(node.args);
     const CChainParams& chainparams = Params();
 
+    SetMockTime(chainparams.GenesisBlock().Time() - 3h);
+
     auto opt_max_upload = ParseByteUnits(args.GetArg("-maxuploadtarget", DEFAULT_MAX_UPLOAD_TARGET), ByteUnit::M);
     if (!opt_max_upload) {
         return InitError(strprintf(_("Unable to parse -maxuploadtarget: '%s'"), args.GetArg("-maxuploadtarget", "")));

</details>

Follow-up to #35351

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36134.

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ACK 55390d1827c724b8c09ac9ffa4b97fccb370b322

ACK 55390d1827c724b8c09ac9ffa4b97fccb370b322

Suggested in perspicacious #35351#pullrequestreview-5066794487 by @mzumsande.