doc: add `-DWITH_BDB=ON` to unix build docs #30779

Existing instructions for building legacy wallet support omit -DWITH_BDB=ON, which results in:

CMake Warning:
  Manually-specified variables were not used by the project:

    BerkeleyDB_INCLUDE_DIR

and a build without BDB support.

This PR updates the docs to include -DWITH_BDB=ON. Also adds a minor correction to the OpenBSD build doc.

Checked by building on Linux (Debian 12.7), FreeBSD 14.1, and OpenBSD 7.5 and attempting to create a legacy wallet with the createwallet rpc (with -deprecatedrpc=create_bdb).

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utACK ddef914bbb1e4fe87e8a85f17e4e839639fd97da

Should we maybe warn in FindBerkeleyDB.cmake when BerkeleyDB_INCLUDE_DIR was set?

Should we maybe warn in FindBerkeleyDB.cmake when BerkeleyDB_INCLUDE_DIR was set?

The <Package>_INCLUDE_DIR are standard CMake variables. And other package Find Modules do not print any warnings. So, introducing any new warning for the BerkeleyDB package would be inconsistent with others.

ACK ddef914bbb1e4fe87e8a85f17e4e839639fd97da