Add document to PHP's complicated RNG backgrounds.

[zeriyoshi]

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7.0 -> 7.1

• srand() / rand() function to alias for the mt_srand() / mt_rand() function.
• But, str_shuffle() function still depends on libc.

7.1 -> 7.2

• Fixed of a modulo bias bug in the MT implementation (some seeds lost compatibility.)
  • ~Need example, trying to implement another MT variations.~ implemented.
• str_shuffle() function still depends on libc.

7.2 -> 7.3

• str_shuffle() function migrated to internal MT functions, no dependency to libc.

[zeriyoshi]

Add Modulo-biased variation #18

[zeriyoshi]

in japanese: https://zenn.dev/zeriyoshi/articles/abd808d1c6d31b