Clarified required OpenGL versions for queries.

Query depends on 3.0/EXT_transform_feedback (also PRIMITIVES_GENERATED,
even if it wasn't in the specs), only part of SampleQuery requires
3.0/NV_conditional_render, the rest is in 2.1 already.

src/Query.h

@@ -101,6 +101,7 @@ if(!q.resultAvailable()) {
 GLuint primitiveCount = q.result<GLuint>();
 @endcode
 @requires_gl
+@requires_gl30 Extension @extension{EXT,transform_feedback}
 */
 class MAGNUM_EXPORT Query: public AbstractQuery {
     public:
@@ -109,8 +110,6 @@ class MAGNUM_EXPORT Query: public AbstractQuery {
             /**
              * Count of primitives generated from vertex shader or geometry
              * shader.
-             *
-             * @requires_gl31 (no extension providing this functionality)
              */
             PrimitivesGenerated = GL_PRIMITIVES_GENERATED,
 
@@ -182,7 +181,6 @@ q.beginConditionalRender(SampleQuery::ConditionalRenderMode::Wait);
 q.endConditionalRender();
 @endcode
 @requires_gl
-@requires_gl30 Extension @extension{NV,conditional_render}
 */
 class MAGNUM_EXPORT SampleQuery: public AbstractQuery {
     public:
@@ -199,7 +197,11 @@ class MAGNUM_EXPORT SampleQuery: public AbstractQuery {
             AnySamplesPassed = GL_ANY_SAMPLES_PASSED
         };
 
-        /** @brief Conditional render mode */
+        /**
+         * @brief Conditional render mode
+         *
+         * @requires_gl30 Extension @extension{NV,conditional_render}
+         */
         enum class ConditionalRenderMode: GLenum {
             /**
              * If query result is not yet available, waits for it and
@@ -236,12 +238,20 @@ class MAGNUM_EXPORT SampleQuery: public AbstractQuery {
         /** @copydoc Query::end() */
         void end();
 
-        /** @brief Begin conditional rendering based on result value */
+        /**
+         * @brief Begin conditional rendering based on result value
+         *
+         * @requires_gl30 Extension @extension{NV,conditional_render}
+         */
         inline void beginConditionalRender(ConditionalRenderMode mode) {
             glBeginConditionalRender(query, static_cast<GLenum>(mode));
         }
 
-        /** @brief End conditional render */
+        /**
+         * @brief End conditional render
+         *
+         * @requires_gl30 Extension @extension{NV,conditional_render}
+         */
         inline void endConditionalRender() {
             glEndConditionalRender();
         }