Add information on how time average mean works

[peterfpeterson]

As part of the event filtering work, this improvement to the event filtering concept page is meant to help the user better understand how mantid calculates time average values.

To test:

Refs #34794

This does not require release notes because it is adding documentation on how time-average values work.

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Reviewer

Please comment on the following (full description):

Code Review

• Is the code of an acceptable quality?
• Does the code conform to the coding standards?
• Are the unit tests small and test the class in isolation?
• If there is GUI work does it follow the GUI standards?
• If there are changes in the release notes then do they describe the changes appropriately?
• Are the release notes saved in a separate file, using Issue or PR number for file name and in the correct location?

Functional Tests

• Do changes function as described? Add comments below that describe the tests performed?
• Do the changes handle unexpected situations, e.g. bad input?
• Has the relevant (user and developer) documentation been added/updated?

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