Add placeholder text info to text input guidance

[EoinShaughnessy]

Fixes #2149.

Updates our text input guidance to give more detail on why users should not use placeholder text.

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[jbuller]

May I suggest a rearrangement and rewording, something like:

All text inputs must have visible labels. Labels should be aligned above the text input they refer to. They should be short, direct and written in sentence case. Do not use colons at the end of labels.
Do not use placeholder text, because:...

• text vanishes when users start typing, which can cause difficulty for people with memory conditions or when reviewing answers.
• browser default styles for placeholder text often do not meet minimum contrast requirements
• not all screen readers read out placeholder

[EoinShaughnessy]

@davidc-gds @edwardhorsford @jbuller Thanks for your comments and suggestions! I've added a commit that tries to combine your feedback.

[jbuller]

I'm sure you're delighted to get this over the line at last and I'm so pleased to have contributed too :)
I do wonder though if the Placeholder bit deserves its own heading?
I know I've pointed people to this guidance in the past and struggled to find it because it lacked a heading.

[EoinShaughnessy]

@jbuller Thanks for your help! We've now added a heading for placeholder text.