doc: document command candidates and the duplicate title rule

The migration guide covered a stanza at a time, which is the wrong
shape for the two idioms that repeated a whole stanza.  One of them,
several candidate binaries for one service, is now a command list.
The other, one service gated differently per platform, has no block
equivalent: those blocks share an identity because they share the
barrier condition downstream services wait for, so they cannot be
given separate titles.  For that one the guide says to split the
variants across files, or leave that file in the line-based format,
which Finit still reads.

The udevd example in services.md taught the merge-broken form, and
system/10-hotplug.conf.in pointed readers at it for their syslogd.

Also lists libConfuse among the build dependencies.  It has been
mandatory since the new .conf format landed, and build.md still said
two libraries.  And corrects the note on variable expansion: it is
${VAR} that libconfuse expands when the file is read, with
${VAR:-default} supported.  A plain $VAR reaches the service, which is
what makes `command = "syslogd -F $SYSLOGD_ARGS"` work with envfile.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/build.md

@@ -2,10 +2,11 @@ Building Finit
 ==============
 
 Finit comes with a traditional configure script to control features and
-optional plugins to enable.  It depends on two external libraries:
+optional plugins to enable.  It depends on three external libraries:
 
 - [libuEv][], the event loop
 - [libite][] (-lite), much needed frog DNA
+- [libConfuse][], the parser behind the .conf format
 
 > [!IMPORTANT]
 > Most free/open source software packages that use `configure` default
@@ -22,6 +23,7 @@ version of the above mentioned libraries.  Currently required versions:
 
 - libite v2.2.0, or later
 - libuEv v2.2.0, or later
+- libConfuse v3.3, or later
 
 
 Configure
@@ -193,3 +195,4 @@ it only for debugging start up issues when Finit crashes.
 [1]:       https://github.com/finit-project/finit/releases/download/4.3/finit-4.3.tar.gz
 [libuEv]:  https://github.com/troglobit/libuev
 [libite]:  https://github.com/troglobit/libite
+[libConfuse]: https://github.com/libconfuse/libconfuse