Add PHP to Zephir string conversion requirements

[ondrejmirtes]

Added critical documentation about converting PHP strings to Zephir:

Key Addition:
When converting PHP code to Zephir, all single-quoted strings must be converted to double-quoted strings and properly escaped following the same escape rules as double-quoted strings in PHP.

Why This Matters:

• In PHP: Both 'text' and "text" create strings
• In Zephir: 'x' creates a char, "text" creates a string
• Single quotes in Zephir are exclusively for single characters (char type)

Changes Made:

1. Dynamic Types Section:

   • Added note that strings must use double quotes (unlike PHP)
   • Added "PHP to Zephir" conversion reminder

2. String vs Char Section:

   • Added dedicated "Important for PHP to Zephir Conversion" subsection
   • Provided side-by-side PHP vs Zephir code examples
   • Showed escape sequence handling differences

3. Common Pitfalls:

   • Added conversion reminder to string vs char confusion item

This prevents a common mistake when porting PHP code to Zephir where developers might try to use single quotes for strings, which would create char types instead.