Two-step verification

A better explanation of the two-way verification.
Some typos.

Correction of some typos.
typo

Colon in dict required
Typu

false -> False
Dropbox

An improved explanation.
Revert "Correction of italic text"

This reverts commit c37d668cdfbaf7d30103e1330d386278f52426ac.
Description of the Field constructor

I propose to move the "Field types" to a separate section to make it
better readable.
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Revert "Revert "Correction of italic text""

This reverts commit 629948fac3155a79d7e2b4cfa9b55c82c4d8d4a0.