Merge #14364: doc: Clarify -blocksdir usage

ccc27bdcd2 doc: Clarify -blocksdir usage (Daniel McNally)

Pull request description:

  This PR attempts to clarify and correct the `-blocksdir` argument description and default value. `-blocksdir` does not refer to the full path to the actual `blocks` directory, but rather the root/parent directory which contains the `blocks` directory. Accordingly, the default value is `<datadir>` and not `<datadir>/blocks` - this behavior of defaulting to the datadir can also be seen in init.cpp:

  ```cpp
      if (gArgs.IsArgSet("-blocksdir")) {
          path = fs::system_complete(gArgs.GetArg("-blocksdir", ""));
          if (!fs::is_directory(path)) {
              path = "";
              return path;
          }
      } else {
          path = GetDataDir(false);
      }
  ```

  It also attempts to clarify that only the `.dat` files containing block data are impacted by `-blocksdir`, not the index files.

  I believe this would close #12828.

ACKs for commit ccc27b:
  hebasto:
    utACK ccc27bdcd2

Tree-SHA512: 7b65f66b0579fd56e8c8cd4f9f22d6af56181817762a68deccd7fca51820ad82d9a0c48f5f1f012e746c67bcdae7af4555fad867cb620a9ca538d465c9d86c2b

src/init.cpp

@@ -380,7 +380,7 @@ void SetupServerArgs()
     gArgs.AddArg("-version", "Print version and exit", false, OptionsCategory::OPTIONS);
     gArgs.AddArg("-alertnotify=<cmd>", "Execute command when a relevant alert is received or we see a really long fork (%s in cmd is replaced by message)", false, OptionsCategory::OPTIONS);
     gArgs.AddArg("-assumevalid=<hex>", strprintf("If this block is in the chain assume that it and its ancestors are valid and potentially skip their script verification (0 to verify all, default: %s, testnet: %s)", defaultChainParams->GetConsensus().defaultAssumeValid.GetHex(), testnetChainParams->GetConsensus().defaultAssumeValid.GetHex()), false, OptionsCategory::OPTIONS);
-    gArgs.AddArg("-blocksdir=<dir>", "Specify blocks directory (default: <datadir>/blocks)", false, OptionsCategory::OPTIONS);
+    gArgs.AddArg("-blocksdir=<dir>", "Specify directory to hold blocks subdirectory for *.dat files (default: <datadir>)", false, OptionsCategory::OPTIONS);
     gArgs.AddArg("-blocknotify=<cmd>", "Execute command when the best block changes (%s in cmd is replaced by block hash)", false, OptionsCategory::OPTIONS);
     gArgs.AddArg("-blockreconstructionextratxn=<n>", strprintf("Extra transactions to keep in memory for compact block reconstructions (default: %u)", DEFAULT_BLOCK_RECONSTRUCTION_EXTRA_TXN), false, OptionsCategory::OPTIONS);
     gArgs.AddArg("-blocksonly", strprintf("Whether to operate in a blocks only mode (default: %u)", DEFAULT_BLOCKSONLY), true, OptionsCategory::OPTIONS);