intro: reword, reference chapter on boot environments

Author: orangecms

src/intro.md

@@ -4,12 +4,12 @@
 
 The Platform System Interface Specification, or _psi-spec_, is
 a collection of documents for [hardware and software co-design](
-#) from a general perspective. It describes how microprocessors
-set up an environment for an operating system or otherwise
-bare-metal applications, typically through intermediate,
-rewritable code that initializes clocks, DRAM and peripherals,
-commonly referred to as boot firmware or [platform initialization firmware](
-platform-initialization.md).
+#) from a general perspective.
+It describes how microprocessors can provide an [environment for booting](
+boot-environments.md) an operating system or otherwise bare-metal applications,
+typically starting from intermediate, rewritable code that initializes clocks,
+DRAM and peripherals, commonly referred to as boot firmware or [platform
+initialization firmware](platform-initialization.md).
 
 In the following sections, _psi-spec_ describes principles and
 abstractions that apply agnostic of vendors and products.