Checking the way the correlations provided with different validity ranges have been selected

[lelaus]

This issue is related to the first task of the "Ranges of Validity" section in issue #147.

The reference handbook provides multiple validity ranges for some correlations. This aspect has been discussed together with the authors of the handbook. Together with them, it has been decided to adopt in lbh15 the lowest range. After having analysed the old issue(s) dealing with this aspect, the corresponding information should be detailed in the "Ranges of Validity" section of the doc.

[panDanieleN]

Communication with authors of the handbook are not related to this section of the doc. As explained in #44 they are related to the choice of some default correlations to adopt in lbh15. Going to share with @lelaus the summary of that communications.

Therefore, I just rephrased the section and followed the suggestions given in #147.

[lelaus]

Documentation modified as described by commits.

No need to update tests.