Update docfx.exe_user_manual.md (#6403)
Update docfx.exe_user_manual.md (#6403)

* Update docfx.exe_user_manual.md

Added information about paths for docfx build and docfx.json src and dest fields to add clarity.

* Update docfx.exe_user_manual.md

Co-authored-by: Yufei Huang <8cdf0fc20d241c10a93fd450af79576eb6093449@live.com>
superyyrrzz/docfx,dotnet/docfx,dotnet/docfx,superyyrrzz/docfx,superyyrrzz/docfx,dotnet/docfx