Fix/update operators.precedence

[guilliamxavier]

• explain why associativity is not applicable to unary operators
• distinguish between unary and binary +/-
• add some missing reference links
• update the precedence of string concatenation for PHP 8
• fix "(n/a)" ("not applicable") to "non-associative" for ternary

References:

• https://github.com/php/php-src/blob/master/Zend/zend_ast.c
  (search "Operator Precedence")
• https://github.com/php/php-src/blob/master/Zend/zend_language_parser.y
  (search "precedence")
  (NOTE: in opposite order)

[guilliamxavier]

Notes:

• Actually I had started with just "update the precedence of string concatenation for PHP 8" but that triggered a cascade of related small changes 😅
• About the table layout: before and after the changes 🤔 Maybe we could add a style white-space: nowrap to the first two columns (result), or insert some line breaks (example)... WDYT?

[cmb69]

Thank you for the very good PR! I'm going to merge it.

About the table layout: before and after the changes 🤔 Maybe we could add a style white-space: nowrap to the first two columns (result), or insert some line breaks (example)... WDYT?

Many tables in the manual have issues (e.g. https://www.php.net/manual/en/filter.filters.validate.php overflows in small browser windows), and often, in my opinion, the best solution would be to replace the tables with lists. In this case I think it might be best to link the operators in the 2nd column directly and add availability info there, and to drop the third column altogether.