docs(manual): Improve line breaks in "SILE Settings" chapter
docs(manual): Improve line breaks in "SILE Settings" chapter

A bunch of small fixes:
- Scope typesetter.parfillskip changes properly (previous resetting
  it to "0 plus 10000pt" is not nearly equivalent as the normal
  infinite fill glue, and affected the rest of the document
  afterwards)
- Use autodoc:command and autodoc:settings when possible, as these
  have provision for extra breakpoints (plus help having an
  homogeneous presentation)

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