docs: document memory ownership and lifecycle in C-API

[markbus-ai]

The current C-API documentation in capi.h is insufficient regarding memory management and object lifecycle, often leading to memory leaks or segmentation faults when implemented in C or via FFI (like Python's ctypes).

This PR updates the Doxygen comments to:

• Clarify Memory Ownership: Explicitly state that functions returning char* (such as TessBaseAPIGetUTF8Text, TessBaseAPIGetHOCRText, TessBaseAPIGetAltoText, etc.) return newly allocated memory that must be freed by the caller using TessDeleteText().
• Define Object Lifecycle: Added a clear, step-by-step lifecycle guide in the TessBaseAPICreate() documentation to help developers understand the mandatory sequence (Create -> Init -> SetImage -> Process -> Delete).
• Leptonica Integration: Clarified that TessBaseAPISetImage2() does NOT take ownership of the Pix structure, and the caller remains responsible for calling pixDestroy().
• Initialization Details: Added information about TessBaseAPIInit3() behavior when datapath is NULL.

These changes improve the developer experience and system stability for anyone using the Tesseract C-API.

[stweil]

Thank you!