docs: "pointer to context object" vs "context object"

[real-or-random]

We should always say "pointer to a secp256k1 context object" in the API docs

From IRC discussion, see https://gnusha.org/secp256k1/2024-01-04.log :

08:37 < nickler> "ctx: pointer to a context object." or "ctx: a secp256k1 context object.". We currently use both arbitrarily afaics.
08:42 < real_or_random> nickler: I think this confusion stems from _context_create where the docs say "Returns: a newly created context object."
08:44 < real_or_random> I guess the initial idea was that the pointers of type secp256k1_context* ARE "context objects" (and don't POINT TO context object). perhaps this is also the reason for the different spacing around the asterisk for pointers 
08:44 < real_or_random> I mean for contexts 
08:46 < real_or_random> but I think that's too much abstraction. we should just always say "pointer to a context object" 
08:46 < sipa> I can't remember ever thinking that the pointer itself was to be treated as the context object.
08:47 < sipa> I think there are other places in the code where we confuse (for brevity) objects and pointers to them, though perhaps not in the public headers.
08:47 < real_or_random> ok, maybe this is just my wrong interpretation