To comment on the source code located in the code section of the page properties of the Reporting Services.rdl file, simply place the apostrophe in front like this.
'ADD YOUR CODE HERE
Take a look at the following article about SQL Server Central:
Adding custom code to Reporting Services
You can also use the apostrophe to post comments in an expression with Reporting Services controls. This only works if comments appear after the definition of the expression. For instance:
= "Test" 'Add Your comments here
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