ut/user_guide: Add explicit reference of clang-format

When mocks are required for module unit testing, it is necessary
to add an explicit clang-format configuration withn the mocks
folder to disable the formatting.

Reflect this explicitly in the user_guide.

Change-Id: Ic9510351f8f23f0e9f85bcaa3234e0a093dc8a7b
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>