DOC: Update variables a and b to names consistent with comment documentation

[migelogali]

Pandas version checks

• I have checked that the issue still exists on the latest versions of the docs on main here

Location of the documentation

https://github.com/pandas-dev/pandas/blob/main/pandas/core/computation/expressions.py

Documentation problem

Lines 234 and 235 explain what a and b are in detail (left and right operands), but there are many of those same variables earlier in the file, making it harder to understand what they represent.

Suggested fix for documentation

Assuming a and b represent right and left operands throughout each function, change these variable names to right_op and left_op instead throughout all functions to have more descriptive variable names

[mingjji]

take