Add documentation on logical operators

[pazathoth]

The manual states

Notice the C++-like syntax; the user needs not study C++ for using FreeFEM, but it helps to guess what is allowed in the language.

and the language goes to great lengths to replicate C++ syntax. Yet, nowhere on an impressive number of PDF pages of documentation it is mentioned that the operators && and || do not use short-circuit evaluation as in C++. One common use case would be to check whether a real variable is zero before division since this results in a runtime error instead of a recoverable infinite value. Therefore I suggest to add a corresponding section in the documentation.

[franckl]

Hello @pazathoth, would you have time to make a pull request ? Your contribution would be greatly appreciated