Make the use of textobject role="description" mandatory for informalfigure as well

[janajaeger]

See $SUBJ. SEO complaints hit us both for informalfigure and figure when used without a description or title that can be used as an alt attribute for HTML. Please rephrase the Figures section to make this mandatory for both types of figures. This is not, however, enforced by the tools in any way. For now, we rely on common sense ;)

[dariavladykina]

Here's the suggested update:

All referenced image files must have a lowercase alphanumeric file name. Provide an appropriate
image width using the width attribute. For both figure types, formal and informal, always add a textobject role="description" as in Example 10, “Example of a figure” to provide an alternative text for the HTML output.

[janajaeger]

Updated version reads nice!

[dariavladykina]

We followed up on this task in #193