Export File: Remove outdated screenshots; use proper Docbook syntax

Screenshots were outdated: No more "Save for Web..." menuitem. Blurry obfuscated text is not a common concept to highlight variables.

Screenshots only depicted a single menu. This is sufficiently self-explanatory given proper documentation text. No complexity level which justifies adding more work for translators.

Also use proper Docbook syntax.

Also add missing articles to sentences.