[TM-224] Mention MText constraints in autodoc

Description

When writing instance TypeHasDoc MText I couldn't refer to the official Michelson tutorial on string literals because constraints listed there didn't correspond to real life.

Recently this MR has been merged which fixes the situation. Site with the tutorials for Zeronet and Alphanet is still not updated, but I hope it will be soon (apparently not later than 005 update comes to life).

So in this commit I add reference to that section of the tutorial.

Related issue(s)

https://issues.serokell.io/issue/TM-224

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