Provide some diagnostic warnings if references to functions are missing parentheses

[vyasr]

Given the following doxygen documentation:

/**
 * @brief Getter function
 *
 * See also `setter`.
 */
void getter();

/**
 * @brief Setter function
 *
 * See also `getter`.
 */
void setter();

Both of the See also lines will fail to link in the built Breathe docs because the reference is missing a set of parentheses to identify this as a function. However, in my documentation I observe that Sphinx also does not generate the usual warnings about a reference target not being found. I have ensured that there is no function attached to the missing-reference event, which suggests that somehow Sphinx is identifying this as an identifier but later silently fails to link. Is there any way to get more useful diagnostics printed here so that we can fix the erroneous references?

[vyasr]

It occurs to me that using single backticks also defaults to non-crosslinking code font in some Sphinx contexts where nothing is inferred, and that also produces no warnings, so this feature may be out of scope for Breathe.