Merge #10315

10315: Fix title of User Manual page r=lnicola a=terrynsun

Asciidoc uses the first line of a doc as the title, so you can't put
comments before the heading.

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I had an old copy of the Manual page open and I noticed that the "User Manual" heading (and page title) had disappeared in the live version:

![image](https://user-images.githubusercontent.com/2858750/134455658-3885b754-9eae-4dca-a6d4-4b1453910850.png)

With this change (and how it was a few days ago):

![image](https://user-images.githubusercontent.com/2858750/134455865-5acc7538-ccf8-4346-941f-3a5561b3181e.png)

Co-authored-by: Terry Sun <terrynsun@gmail.com>