docs: document differences between config_lookup and typed lookups (#242)

[maruixin-kls]

Summary

This PR updates the documentation to clarify the distinction and historical context between path-based lookups (config_lookup) and typed lookups (config_lookup_int, etc.), as requested in issue #242.

Changes Made

• Added a section in libconfig.texi explaining that config_lookup returns a pointer to the setting structure, allowing for further traversal or type checking.
• Clarified that typed lookup functions (e.g., config_lookup_int) are convenience wrappers that combine path resolution and type conversion into a single step.
• Highlighted the difference in return values (pointer vs. boolean status) which is a common point of confusion for users migrating from older patterns.

Why this is important

Users often confuse when to use the generic lookup versus the typed lookup. Documenting this "historical" design choice helps clarify that typed lookups are essentially syntactic sugar for config_lookup followed by setting_get_*, while the generic lookup offers more flexibility for complex structures.

Fixes #242

[hyperrealm]

Thanks for adding this!