Update ZJIT docs

Author: tekknolagi

NEWS.md

@@ -384,7 +384,7 @@ A lot of work has gone into making Ractors more stable, performant, and usable.
 ## JIT
 
 * ZJIT
-    * Introduce an experimental method-based JIT compiler.
+    * Introduce an [experimental method-based JIT compiler](https://docs.ruby-lang.org/en/master/jit/zjit_md.html).
       To enable `--zjit` support, build Ruby with Rust 1.85.0 or later.
     * As of Ruby 4.0.0, ZJIT is faster than the interpreter, but not yet as fast as YJIT.
       We encourage experimentation with ZJIT, but advise against deploying it in production for now.

doc/jit/zjit.md

@@ -1,7 +1,49 @@
 # ZJIT: ADVANCED RUBY JIT PROTOTYPE
 
+ZJIT is a method-based just-in-time (JIT) compiler for Ruby. It uses profile
+information from the interpreter to guide optimization in the compiler.
+
+ZJIT is currently supported for macOS, Linux and BSD on x86-64 and arm64/aarch64 CPUs.
+This project is open source and falls under the same license as CRuby.
+
+## Current Limitations
+
+ZJIT may not be suitable for certain applications. It currently only supports macOS, Linux and BSD on x86-64 and arm64/aarch64 CPUs. ZJIT will use more memory than the Ruby interpreter because the JIT compiler needs to generate machine code in memory and maintain additional state information.
+You can change how much executable memory is allocated using [ZJIT's command-line options](#command-line-options).
+
 ## Build Instructions
 
+### For normal use
+
+To build ZJIT on macOS:
+
+```bash
+./autogen.sh
+
+./configure \
+    --enable-zjit \
+    --prefix="$HOME"/.rubies/ruby-zjit \
+    --disable-install-doc \
+    --with-opt-dir="$(brew --prefix openssl):$(brew --prefix readline):$(brew --prefix libyaml)"
+
+make -j miniruby
+```
+
+To build ZJIT on Linux:
+
+```bash
+./autogen.sh
+
+./configure \
+    --enable-zjit \
+    --prefix="$HOME"/.rubies/ruby-zjit \
+    --disable-install-doc
+
+make -j miniruby
+```
+
+### For development
+
 To build ZJIT on macOS:
 
 ```bash