User manual proof reading till "2.3.8 Styles"

Proof reading mostly done by @michleb !
The manual generation is fine; tell me if anything is wrong.

```
$ make darktable-usermanual-fr
[  0%] Generating the profiled docbook xml file
[  0%] Built target darktable_profile_xml
[100%] Built target target_media_images
[100%] Generating the main docbook xml file
[100%] Building fr usermanual
Making portrait pages on A4 paper (210mmx297mm)
Font "Symbol,normal,700" not found. Substituting with "Symbol,normal,400".
Font "ZapfDingbats,normal,700" not found. Substituting with "ZapfDingbats,normal,400".
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Line 1 of a paragraph overflows the available area by more than 50 points. (See position 930:27795)
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span="inherit" on fo:block, but no explicit value found on the parent FO.
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[100%] Built target darktable-usermanual-fr

```