Change the default sampler to ParentOrElse(AlwaysOn) (open-telemetry#750)

* Change the default sampler to ParentOrElse(AlwaysOn)

* addressed some review comments

* more review feedback

Author: MitchellDumovic

specification/trace/sdk.md

@@ -49,7 +49,7 @@ The SDK defines the interface [`Sampler`](#sampler) as well as a set of
 
 ### Sampler
 
-`Sampler` interface allows to create custom samplers which will return a
+`Sampler` interface allows users to create custom samplers which will return a
 sampling `SamplingResult` based on information that is typically available just
 before the `Span` was created.
 
@@ -94,10 +94,11 @@ be displayed on debug pages or in the logs. Example:
 Description MUST NOT change over time and caller can cache the returned value.
 
 ### Built-in samplers
+OpenTelemetry supports a number of built-in samplers to choose from. 
+The default sampler is `ParentOrElse(AlwaysOn)`.
 
 #### AlwaysOn
 
-* This is the default sampler.
 * Returns `RECORD_AND_SAMPLED` always.
 * Description MUST be `AlwaysOnSampler`.