[DOC] Refine/fix doc/contributing/building_ruby.md [ci skip]

* Separate dependencies for RubyGems
* Add reasons of optional libraries
* Add a note for gperf and fix the required version

doc/contributing/building_ruby.md

@@ -5,6 +5,9 @@
 1. Install the prerequisite dependencies for building the CRuby interpreter:
 
     * C compiler
+
+    For RubyGems, you will also need:
+
     * OpenSSL 1.1.x or 3.0.x / LibreSSL
     * libyaml 0.1.7 or later
     * zlib
@@ -13,14 +16,15 @@
 
     * autoconf - 2.67 or later
     * bison - 3.0 or later
-    * gperf - 3.0.3 or later
+    * gperf - 3.1 or later
+        * Usually unneeded; only if you edit some source files using gperf
     * ruby - 2.2 or later
         * We can upgrade this version to system ruby version of the latest Ubuntu LTS.
 
 2. Install optional, recommended dependencies:
 
-    * readline/editline (libedit)
-    * libffi
+    * readline/editline (libedit, to build readline)
+    * libffi (to build fiddle)
     * gmp (if you with to accelerate Bignum operations)
     * libexecinfo (FreeBSD)
     * rustc - 1.58.0 or later (if you wish to build [YJIT](/doc/yjit/yjit.md))