docs: document on_entry<_> multi-TU limitation and dispatch priority (#565)

[PavelGuzenfeld]

Summary

Closes #565 (documentation fix — no library behaviour change).

What was happening

on_entry<_> maps to a generic on_entry<_, _> handler that is instantiated for every event that can reach the state, including the internal back::initial and back::anonymous events. When the action has a generic (auto/template) call operator, each event type generates a distinct template specialisation. Placing only a declaration in a header and defining the body in a separate .cpp leaves the initial-event specialisation with no definition → linker error even though the code looks correct.

Changes

include/boost/sml.hpp — comment block added above the on_entry struct:

• Explains the instantiation-per-event behaviour
• Explains why multi-TU generic actions fail (linker "undefined reference" for initial specialisation)
• Lists three workarounds: define inline, use on_entry<E> with a specific type, or use a non-generic call operator

test/ft/states.cpp — new test on_entry_wildcard_fires_for_all_events:

• Adds s3 state at file scope (needed by the test)
• Verifies dispatch priority: when both on_entry<_> and on_entry<e1> are registered for the same state, the specific handler fires for e1 and the wildcard is the fallback for events without a specific handler (here e2)

Verified

• GCC C++14 / C++17 / C++20: all tests pass
• MSVC C++20: compiles cleanly (cl.exe /W4 /WX)