doc: clarify ast_always_print_block option
doc: clarify ast_always_print_block option

In particular, clarify that it only applies to bodies
of if and for nodes.
Note that the help message of the option already mentions this.

Signed-off-by: Sven Verdoolaege <dd860110a62b19214c4ee03aec0abffecb4e86b8@cerebras.net>

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