Clarify master and single directives.

A customer reported that the Remarks section for the master and single directives was confusing. Updated to make it clear that the reference to single in the remarks for master and the reference to master in the remarks for single are suggesting alternatives.

Have to use 'master' instead of 'main' here because that it is an OpenMP keyword.

Author: branh

docs/parallel/openmp/reference/openmp-directives.md

@@ -369,10 +369,10 @@ Specifies that only the master thread should execute a section of the program.
 
 The `master` directive supports no clauses.
 
-The [single](#single) directive lets you specify that a section of code should be executed on a single thread, not necessarily the master thread.
-
 For more information, see [2.6.1 master construct](../2-directives.md#261-master-construct).
 
+To specify that a section of code should be executed on a single thread, not necessarily the master thread, use the [single](#single) directive instead.
+
 ### Example
 
 ```cpp
@@ -629,10 +629,10 @@ The `single` directive supports the following clauses:
 - [copyprivate](openmp-clauses.md#copyprivate)
 - [nowait](openmp-clauses.md#nowait)
 
-The [master](#master) directive lets you specify that a section of code should be executed only on the master thread.
-
 For more information, see [2.4.3 single construct](../2-directives.md#243-single-construct).
 
+To specify that a section of code should only be executed on the master thread, use the [master](#master) directive instead.
+
 ### Example
 
 ```cpp