gh-36841: Change default domain of SR.var()

    
This patch fixes #36833 by changing the default domain of SR.var()
function to complex. By this way, every variable whose domain is not
defined will be considered
in complex plane, by default.

The issue was with the default value only because whenever we were
defining a variable without specifying any domain it was calling
symbol() function with domain=None; Subsequently, it calls
new_Expression_symbol() function with domain=None. In this code, I found
that it was not giving any specific domain to the variable and so when
we define a new variable using SR.var() function, it is passing the
is_real() (Because it does not have any specific domain) test but giving
the wrong answer on factor() function, if we assume that the variable is
in complex plane by default which is correct according to the
documentation.

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URL: #36841
Reported by: Ruchit Jagodara
Reviewer(s): Dima Pasechnik, Ruchit Jagodara

src/sage/symbolic/ring.pyx

@@ -760,7 +760,7 @@ cdef class SymbolicRing(sage.rings.abc.SymbolicRing):
 
         - ``n`` -- (optional) positive integer; number of symbolic variables, indexed from `0` to `n-1`
 
-        - ``domain`` -- (optional) specify the domain of the variable(s); it is the complex plane
+        - ``domain`` -- (optional) specify the domain of the variable(s); it is None
           by default, and possible options are (non-exhaustive list, see note below):
           ``'real'``, ``'complex'``, ``'positive'``, ``'integer'`` and ``'noninteger'``
 
@@ -780,7 +780,7 @@ cdef class SymbolicRing(sage.rings.abc.SymbolicRing):
 
         EXAMPLES:
 
-        Create a variable `zz` (complex by default)::
+        Create a variable `zz`::
 
             sage: zz = SR.var('zz'); zz
             zz