Update for the pt => px conversion section

[ArnaudDelafosse]

Tried my best to keep it clear for users, feel free to modify at will or completely ignore :) Also did a couple of syntax corrections and made the titles match the app labels for clarity.

I verified my work is based on develop branch

• [x] yes, keep going

[dzc34]

related to #136

[dzc34]

Thanks @ArnaudDelafosse for your contributing! I splited your merge request in two part for more easy review:

• 142 - DOC - Howto-use.md improvements (syntax corrections, match titles to application labels)

• 143 - DOC - Howto-use.md / Update for the pt => px conversion section

142 is now merged in develop branch.

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Only one change was not include because "Other info" is part of "Minimum ratio"

-### Other info
+## Other info