The excerpt search documentation page is slightly misleading about some of the parameters, which are stated to pertain to questions only:
q- a free form text parameter, will match all question properties based on an undocumented algorithm.
body- text which must appear in returned questions' bodies.
user- the id of the user who must own the questions returned.
wiki- true to return only community wiki questions, false to return only non-community wiki ones. Omit to elide constraint.
In all cases, 'question(s)' should be replaced with 'post(s)'. This is probably a copy/paste from the advanced search method which searches in questions only.