fix doxygen for XLALVectorComputeGLtLDenominator

Description

@karl-wette minor fix for an "Undefined control sequence" and providing a proper sentence for the function along the way.

The whole file is kinda messy LaTeX-wise with some commands being redefined per function and others being reused, but a first brute-force cleanup attempt failed, and I don't want to spend much time on making it nicer for now. So just this isolated fix for now.

API Changes and Justification

Backwards Compatible Changes

• This change does not modify any class/function/struct/type definitions in a public C header file or any Python class/function definitions
• This change adds new classes/functions/structs/types to a public C header file or Python module

Backwards Incompatible Changes

• This change modifies an existing class/function/struct/type definition in a public C header file or Python module
• This change removes an existing class/function/struct/type from a public C header file or Python module

If any of the Backwards Incompatible check boxes are ticked please provide a justification why this change is necessary and why it needs to be done in a backwards incompatible way.

Review Status

n/a

lalpulsar/lib/LineRobustStats.c

@@ -237,7 +237,12 @@ XLALComputeBSGL ( const REAL4 twoF,				//!< [in] multi-detector F-stat \f$ 2\F \
 
 } // XLALComputeBSGL()
 
-// module-internal function
+/**
+ * \f[
+ * \newcommand{\cohF}{\coh{\F}}
+ * \f]
+ * Helper function to compute the denominator for XLALVectorComputeBSGLtL() and XLALVectorComputeBtSGLtL() - see their documentation for details.
+ */
 int
 XLALVectorComputeGLtLDenominator ( REAL4 *outDenom,					//!< [out] pre-allocated output array of denominator values
                                    const REAL4 *twoFPerDet[PULSAR_MAX_DETECTORS],	//!< [in] input vector of semi-coherent sums \f$ \{2\scF^X\} \f$ of per-detector F-stats