Update ob_get_status and ob_start documentation

[lacatoire]

ob_get_status() documents buffer_size as "Output buffer size in bytes", which says nothing about what the number means or why it moves. The linked issue also asks why chunk_size of 0 and 1 produce different buffer_size values.

The allocation rule is in main/php_output.h:

#define PHP_OUTPUT_HANDLER_INITBUF_SIZE(s) \
( ((s) > 0) ? ZEND_MM_ALIGNED_SIZE_EX(s, PHP_OUTPUT_HANDLER_ALIGNTO_SIZE) \
            : PHP_OUTPUT_HANDLER_DEFAULT_SIZE )
#define PHP_OUTPUT_HANDLER_ALIGNTO_SIZE   0x1000   //  4096
#define PHP_OUTPUT_HANDLER_DEFAULT_SIZE   0x4000   // 16384

Measured on PHP 8.5.4:

chunk_size=0      -> buffer_size=16384
chunk_size=1      -> buffer_size=4096
chunk_size=100    -> buffer_size=4096
chunk_size=4096   -> buffer_size=4096
chunk_size=10000  -> buffer_size=12288

growth with the default chunk_size:
  16000 bytes written -> 16384       16384 -> 32768
  40000 bytes written -> 40960      100000 -> 102400

So the initial size is 16384 with the default chunk_size, not one memory page, and any explicit chunk_size is rounded up to the next multiple of 4096. Both pages now state that.

The flush behaviour of chunk_size = 1 is documented as well, verified with an output callback: echo "A"; echo "B"; echo "C"; reaches the callback as three separate chunks with chunk_size = 1, as a single "ABC" with 0, and as "AB", "CD" with 2.

Fixes: #3816