Update FGFunction documentation

[seanmcleod]

While writing some example Python based code - #1126 (comment) - to process FDM XML files I noticed a small discrepancy between the documentation for function elements in:

jsbsim/src/math/FGFunction.h

Lines 122 to 124 in 651ad3c

	configuration file includes the function element, and other elements. It should
	be noted that there can be only one non-optional (non-documentation) element -
	that is, one operation element - in the top-level function definition.

Versus what I came across in a number of FDM files when testing, e.g.

jsbsim/aircraft/f16/f16.xml

Lines 986 to 1009 in 651ad3c

	<aerodynamics>
	<function name="aero/function/kCLge">
	<description>Change_in_lift_due_to_ground_effect</description>
	<table>
	<independentVar>aero/h_b-mac-ft</independentVar>
	<tableData>
	0.0000 1.2290
	0.1000 1.1240
	0.1500 1.1160
	0.2000 1.1240
	0.3000 1.1050
	0.4000 1.0410
	0.5000 1.0340
	0.6000 1.0190
	0.7000 1.0080
	0.8000 1.0030
	0.9000 1.0010
	1.0000 1.0000
	1.1000 1.0000
	</tableData>
	</table>
	</function>

And also in JSBSim.xsd:

jsbsim/JSBSim.xsd

Lines 1456 to 1464 in 651ad3c

	<xs:element name="function">
	<xs:complexType>
	<xs:sequence>
	<xs:element ref="description" minOccurs="0"/>
	<xs:group ref="func_group" />
	</xs:sequence>
	<xs:attribute name="name" use="optional" />
	</xs:complexType>
	</xs:element>

jsbsim/JSBSim.xsd

Lines 1403 to 1410 in 651ad3c

	<!-- Functions -->
	<xs:group name="func_group">
	<xs:choice>
	<xs:element ref="table" />
	<xs:element ref="product" />
	<xs:element ref="difference" />
	<xs:element ref="sum" />
	<xs:element ref="quotient" />

So this pull request provides a minor update to the comment in the source code for FGFunction.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 24.93%. Comparing base (651ad3c) to head (291519d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1131   +/-   ##
=======================================
  Coverage   24.93%   24.93%           
=======================================
  Files         170      170           
  Lines       18239    18239           
=======================================
  Hits         4548     4548           
  Misses      13691    13691           

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[bcoconni]

Looks good to me.