docs: refine MSER and DoG scale space explanations

[nRanzo]

Description

new q&a for lesson 12

Checklist

Before submitting, please make sure you can check all of these boxes (put an 'x' inside the brackets: [x])

• I used the custom \qa{Question}{Answer} command.
• I did not modify main.tex (unless I am explicitly adding a completely new lecture file).
• I am reasonably sure my LaTeX syntax is correct (watch out for unescaped characters like %, _, or &!).
• The content is relevant to our Computer Vision course.

Any Doubts? (Optional)