DOC: Clarified how finfo works with complex numbers (numpy#22260)

There was some confusion about how finfo works with complex numbers,
leading to an issue being made requesting that numpy either adds a
cinfo function or changes the documentation of finfo. The submitter of
that issue also linked to an issue for a seperate repository, which also
included conversation about the proposed change. (github.com/data-apis/array-api/issues/433)
In both discussions, there was a general concensus that it would be better
to change the documentation to explain how finfo works instead of
creating a cinfo function.

Since this is just a small documentation change, there's no need
to run the normal checks.

[skip ci]

Author: MatteoRaso

numpy/core/getlimits.py

@@ -423,7 +423,8 @@ class finfo:
     Parameters
     ----------
     dtype : float, dtype, or instance
-        Kind of floating point data-type about which to get information.
+        Kind of floating point or complex floating point
+        data-type about which to get information.
 
     See Also
     --------
@@ -445,6 +446,11 @@ class finfo:
     fill the gap between 0 and ``smallest_normal``. However, subnormal numbers
     may have significantly reduced precision [2]_.
 
+    This function can also be used for complex data types as well. If used,
+    the output will be the same as the corresponding real float type
+    (e.g. numpy.finfo(numpy.csingle) is the same as numpy.finfo(numpy.single)).
+    However, the output is true for the real and imaginary components.
+
     References
     ----------
     .. [1] IEEE Standard for Floating-Point Arithmetic, IEEE Std 754-2008,