Merge bitcoin/bitcoin#26773: doc: FreeBSD build doc updates to reflect removal of install_db4.sh

c572eae update the freebsd build doc to reflect recent changes to DB4 install process (Murray Nesbitt)

Pull request description:

  This PR introduces documentation changes needed to keep up with #26834.

ACKs for top commit:
  fanquake:
    ACK c572eae - have not tested, but looks ok.

Tree-SHA512: 42a79e7b45834916b1b738db524b51b9ff4fde8348ba66fc331ff6603532dd9fce73ea392eef97d31112326c6d60ec2c5c7c29e66aab33aaf846aab8aea1d1aa

Author: fanquake

doc/build-freebsd.md

@@ -36,13 +36,30 @@ pkg install sqlite3
 ```
 
 ###### Legacy Wallet Support
-`db5` is only required to support legacy wallets.
-Skip if you don't intend to use legacy wallets.
+BerkeleyDB is only required if legacy wallet support is required.
+
+It is required to use Berkeley DB 4.8. You **cannot** use the BerkeleyDB library
+from ports. However, you can build DB 4.8 yourself [using depends](/depends).
 
-```bash
-pkg install db5
 ```
----
+gmake -C depends NO_BOOST=1 NO_LIBEVENT=1 NO_QT=1 NO_SQLITE=1 NO_NATPMP=1 NO_UPNP=1 NO_ZMQ=1 NO_USDT=1
+```
+
+When the build is complete, the Berkeley DB installation location will be displayed:
+
+```
+to: /path/to/bitcoin/depends/x86_64-unknown-freebsd[release-number]
+```
+
+Finally, set `BDB_PREFIX` to this path according to your shell:
+
+```
+csh: setenv BDB_PREFIX [path displayed above]
+```
+
+```
+sh/bash: export BDB_PREFIX=[path displayed above]
+```
 
 #### GUI Dependencies
 ###### Qt5
@@ -91,12 +108,12 @@ This explicitly enables the GUI and disables legacy wallet support, assuming `sq
 
 ##### Descriptor & Legacy Wallet. No GUI:
 This enables support for both wallet types and disables the GUI, assuming
-`sqlite3` and `db5` are both installed.
+`sqlite3` and `db4` are both installed.
 ```bash
 ./autogen.sh
-./configure --with-gui=no --with-incompatible-bdb \
-    BDB_LIBS="-ldb_cxx-5" \
-    BDB_CFLAGS="-I/usr/local/include/db5" \
+./configure --with-gui=no \
+    BDB_LIBS="-L${BDB_PREFIX}/lib -ldb_cxx-4.8" \
+    BDB_CFLAGS="-I${BDB_PREFIX}/include" \
     MAKE=gmake
 ```