Docs: Add info on disabling phone no. formatting

By default Safari on iOS will automatically inject a `<a href="tel:…">`
when it detects any string formatted like a phone number on the web page
(see: https://cloud.githubusercontent.com/assets/1223565/4230929/90243cd8-398a-11e4-9363-67faf5cb355c.png).

This commit improves the documentation by providing information on
how to disable the automatic detection and formatting of possible phone
numbers in Safari on iOS.

Ref: https://twitter.com/marcosc/status/507897299587313665/
     https://developer.apple.com/library/safari/documentation/AppleApplications/Reference/SafariHTMLRef/Articles/MetaTags.html/#//apple_ref/doc/uid/TP40008193-SW5