Highlight that SECTIONs rerun the entire test case from beginning (#2749

)

docs/tutorial.md

@@ -119,7 +119,7 @@ This is best explained through an example ([code](../examples/100-Fix-Section.cp
 
 ```c++
 TEST_CASE( "vectors can be sized and resized", "[vector]" ) {
-
+    // This setup will be done 4 times in total, once for each section
     std::vector<int> v( 5 );
 
     REQUIRE( v.size() == 5 );
@@ -152,11 +152,12 @@ TEST_CASE( "vectors can be sized and resized", "[vector]" ) {
 }
 ```
 
-For each `SECTION` the `TEST_CASE` is executed from the start. This means
+For each `SECTION` the `TEST_CASE` is **executed from the start**. This means
 that each section is entered with a freshly constructed vector `v`, that
 we know has size 5 and capacity at least 5, because the two assertions
-are also checked before the section is entered. Each run through a test
-case will execute one, and only one, leaf section.
+are also checked before the section is entered. This behaviour may not be
+ideal for tests where setup is expensive. Each run through a test case will
+execute one, and only one, leaf section.
 
 Section can also be nested, in which case the parent section can be
 entered multiple times, once for each leaf section. Nested sections are