[doc] Clarify C Static Declarations
[doc] Clarify C Static Declarations

This disallows static declarations in headers, and moves the discussion
of `static inline` into the section specifically addressing `static`
declarations.

This also codifies that header files are declarations of public
interfaces.

Signed-off-by: Sam Elliott <32a4841301e2f444934d6e427a2764482fa7a43d@lowrisc.org>

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