Improve A.all (#1413)

* reword the A.1 rule title
* add candidate content for the A.1 rule
* make minor improvements to the A.2 note
* simplify wording in the first bullet of A.4's Reason

CppCoreGuidelines.md

@@ -19764,7 +19764,7 @@ This section contains ideas about higher-level architectural ideas and libraries
 
 Architectural rule summary:
 
-* [A.1: Separate stable from less stable part of code](#Ra-stable)
+* [A.1: Separate stable code from less stable code](#Ra-stable)
 * [A.2: Express potentially reusable parts as a library](#Ra-lib)
 * [A.4: There should be no cycles among libraries](#Ra-dag)
 * [???](#???)
@@ -19774,9 +19774,9 @@ Architectural rule summary:
 * [???](#???)
 * [???](#???)
 
-### <a name="Ra-stable"></a>A.1: Separate stable from less stable part of code
+### <a name="Ra-stable"></a>A.1: Separate stable code from less stable code
 
-???
+Isolating less stable code facilitates its unit testing, interface improvement, refactoring, and eventual deprecation.
 
 ### <a name="Ra-lib"></a>A.2: Express potentially reusable parts as a library
 
@@ -19785,15 +19785,15 @@ Architectural rule summary:
 ##### Note
 
 A library is a collection of declarations and definitions maintained, documented, and shipped together.
-A library could be a set of headers (a "header only library") or a set of headers plus a set of object files.
+A library could be a set of headers (a "header-only library") or a set of headers plus a set of object files.
-A library can be statically or dynamically linked into a program, or it may be `#include`d
+You can statically or dynamically link a library into a program, or you can `#include` a header-only library.
 
 
 ### <a name="Ra-dag"></a>A.4: There should be no cycles among libraries
 
 ##### Reason
 
-* A cycle implies complication of the build process.
+* A cycle complicates the build process.
 * Cycles are hard to understand and may introduce indeterminism (unspecified behavior).
 
 ##### Note