ci: speed up code-coverage jobs

some documentation updates

refs #3809

Change-Id: I0c837a3b8f0f5e5e15d20ba41418bf17660fffb5
diff --git a/.jenkins.d/README.md b/.jenkins.d/README.md
index 386e2db..085e383 100644
--- a/.jenkins.d/README.md
+++ b/.jenkins.d/README.md
@@ -9,17 +9,17 @@
 
   The list should include at least `[OS_TYPE]`, `[DISTRO_TYPE]`, and `[DISTRO_VERSION]`.
 
-  Possible values for Linux OS:
+  Possible values for Linux:
 
   * `[OS_TYPE]`: `Linux`
   * `[DISTRO_TYPE]`: `Ubuntu`
   * `[DISTRO_VERSION]`: `Ubuntu-14.04`, `Ubuntu-16.04`
 
-  Possible values of OSX OS:
+  Possible values for OS X / macOS:
 
   * `[OS_TYPE]`: `OSX`
   * `[DISTRO_TYPE]`: `OSX` (can be absent)
-  * `[DISTRO_VERSION]`: `OSX-10.11`, `OSX-10.10`, `OSX-10.9`, `OSX-10.8`
+  * `[DISTRO_VERSION]`: `OSX-10.10`, `OSX-10.11`, `OSX-10.12`
 
 - `JOB_NAME`: optional variable to define type of the job.  Depending on the defined job type,
   the build scripts can perform different tasks.
@@ -27,8 +27,8 @@
   Possible values:
 
   * empty: default build process
-  * `code-coverage` (Linux OS is assumed): build process with code coverage analysis
-  * `limited-build`: only a single build with tests
+  * `code-coverage` (Linux OS is assumed): debug build with tests and code coverage analysis
+  * `limited-build`: only a single debug build with tests
 
 - `CACHE_DIR`: the variable defines a path to folder containing cached files from previous builds,
   e.g., a compiled version of ndn-cxx library.  If not set, `/tmp` is used.