Update RuntimeHelpers.xml (dotnet#6772)

Author: YairHalberstadt

xml/System.Runtime.CompilerServices/RuntimeHelpers.xml

@@ -1228,13 +1228,14 @@
  </Parameters>
  <Docs>
  <param name="module">A handle that specifies the module constructor method to run.</param>
- <summary>Runs a specified module constructor method.</summary>
+ <summary>Ensures a specified module constructor method has run by the time this method returns.</summary>
  <remarks>
  <format type="text/markdown"><![CDATA[ 
  
 ## Remarks 
- This method is used by compilers. 
- 
+ This method is used by compilers.
+
+ The module constructor will not be run more than once, even if this method is called multiple times.
  ]]></format>
  </remarks>
  <exception cref="T:System.TypeInitializationException">The module constructor throws an exception.</exception>