CH03::building: mention daemon & Unix; link to other instructions

- Add just a few words so users know what the "d" in bitcoind and the
  "Unix" in build-unix.md stand for.

- Since the last update to this text, there are instructions for many
  more platforms available, so rewrite final sentence to alert users to
  them.

ch03.asciidoc

@@ -177,12 +177,13 @@ seealso="Bitcoin Core")))The source code includes documentation, which
 can be found in a number of files. Review the main documentation located
 in _README.md_ in the _bitcoin_ directory by typing ++**more
 README.md**++ at the prompt and using the spacebar to progress to the
-next page. In this chapter, we will build the command-line Bitcoin
-client, also known as +bitcoind+ on Linux. Review the instructions for
+next page. In this chapter, we will build the Bitcoin Core daemon
+(server), also known as +bitcoind+ on Linux (a Unix-like system). Review the instructions for
 compiling the +bitcoind+ command-line client on your platform by typing
-++**more doc/build-unix.md**++. Alternative instructions for macOS and
-Windows can be found in the _doc_ directory, as _build-osx.md_ or
-_build-windows.md_, respectively.
+++**more doc/build-unix.md**++.  Alternative instructions can be found in
+the _doc_ directory; for example, _build-windows.md_ for Windows
+instructions.  As of this writing, instructions are available for
+Android, FreeBSD, NetBSD, OpenBSD, MacOS (OSX), Unix, and Windows.
 
 Carefully review the build prerequisites, which are in the first part of
 the build documentation. These are libraries that must be present on