Enhanced RDoc concerning command injection (#5537)

Clarifies security vulnerabilities for commands.

Treats:

    Kernel.system
    Kernel.` (backtick)
    IO.popen
    IO.read
    IO.write
    IO.binread
    IO.binwrite
    IO.readlines
    IO.foreach

doc/command_injection.rdoc

@@ -0,0 +1,29 @@
+== Command Injection
+
+Some Ruby core methods accept string data
+that includes text to be executed as a system command.
+
+They should not be called with unknown or unsanitized commands.
+
+These methods include:
+
+- Kernel.system
+- {`command` (backtick method)}[rdoc-ref:Kernel#`]
+  (also called by the expression <tt>%x[command]</tt>).
+- IO.popen(command).
+- IO.read(command).
+- IO.write(command).
+- IO.binread(command).
+- IO.binwrite(command).
+- IO.readlines(command).
+- IO.foreach(command).
+
+Note that some of these methods do not execute commands when called
+from subclass \File:
+
+- File.read(path).
+- File.write(path).
+- File.binread(path).
+- File.binwrite(path).
+- File.readlines(path).
+- File.foreach(path).