In-place modifications made explicit in docs for encode(), decode() and decode_utf8()

[polettix]

This patch adds a few lines in the docs for encode(), decode() and decode_utf8() to make it explicit to the reader that their inputs might be at risk of being modified.

This information is already contained in the docs, although it's a bit easy to overlook it; the goal of the patch is to make anyone reading the docs for one of those functions immediately aware that their inputs might be at risk unless proper actions are taken.

[dankogai]

Thank you so much!

[polettix]

Thank you for taking care of this module, I guess it's not a simple beast at all but provides an invaluable service to all of us.