Improve g_ascii_formatd docs and preconditions

Matthias Clasenrequested to merge
ascii-formatd-libc-dep into main

Clarify that the format passed to g_ascii_formatd must be just a single format specifier for a double argument, and must start with '%'.

Apply the precondition checks for this regardless which libc we are using.

Related to pango#659 (closed)

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