Merge #18219: doc: Add warning against wallet.dat re-use

c1e0742308 doc: Warn about wallet.dat re-use and backups (Albert)

Pull request description:

  Following discussion in #18205, this PR adds a warning against re-use of the same wallet file on two different nodes, as that can cause problems due to race conditions between nodes (eg: both nodes using the same addresses at the same time for different things because they are not aware of the other node).

  I've also included the rationale behind the warning but I've kept it short to make it clearer to users, not sure if I should have written a longer explanation instead.

  Also, while this PR may help some users avoid problems, the changes are largely inconsequential, so feel free to close it if it's not worth the effort.

  On an unrelated note, I've also set up [this site](https://corollari.github.io/bitcoin-core-docs/), which periodically pulls bitcoin core and turns its docs into a webpage. Browsing the docs can also be done locally or on github, so this doesn't add much value, but I personally find that more comfortable and it makes them more searchable.

Top commit has no ACKs.

Tree-SHA512: 5ce06026176917304932714470be8c3410d35698f925875b0955ecd3b1756ef52793feb469dd4bdac4921f1a24daf59001e9911f1f096f559fb28c250baae378

doc/files.md

@@ -78,6 +78,10 @@ Subdirectory | File(s)           | Description
 
 3. A wallet database path can be specified by `-wallet` option.
 
+4. `wallet.dat` files must not be shared across different node instances, as that can result in key-reuse and double-spends due the lack of synchronization between instances.
+
+5. Any copy or backup of the wallet should be done through a `backupwallet` call in order to update and lock the wallet, preventing any file corruption caused by updates during the copy.
+
 ## GUI settings
 
 `bitcoin-qt` uses [`QSettings`](https://doc.qt.io/qt-5/qsettings.html) class; this implies platform-specific [locations where application settings are stored](https://doc.qt.io/qt-5/qsettings.html#locations-where-application-settings-are-stored).