Document the previously undocumented .Bf (begin font mode) macro. Moved the descriptions of the .Em, .Li and .Sy macros into the new "Font Modes" sub-section along with .Bf, since they are all related.
Document the previously undocumented .Bf (begin font mode) macro.
Moved the descriptions of the .Em, .Li and .Sy macros into
the new "Font Modes" sub-section along with .Bf, since they are
all related.

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