Merge bitcoin/bitcoin#32206: doc: Add fuzz based coverage report generation

7677fde4c7 Add fuzz test coverage report generation (Prabhat Verma)

Pull request description:

  Followup from [comment 1 31933](https://github.com/bitcoin/bitcoin/pull/31933#pullrequestreview-2685168897) and [comment 2 31933](https://github.com/bitcoin/bitcoin/pull/31933#pullrequestreview-2683653107) , have added the instructions to generate coverage report for fuzz tests.

ACKs for top commit:
  maflcko:
    lgtm ACK 7677fde4c7
  mabu44:
    Tested ACK 7677fde4c7

Tree-SHA512: b4883ec41e272443fc9fc97dce3fb0f05871ce5ddc07d97f4cf7e1d28506dc892324b42005387a2f5a844b27827785a68d10881067414fe93fd4fbb9645f765d

doc/developer-notes.md

@@ -575,6 +575,47 @@ llvm-cov show \
 
 The generated coverage report can be accessed at `build/coverage_report/index.html`.
 
+#### Compiling for Fuzz Coverage
+
+```shell
+cmake -B build \
+   -DCMAKE_C_COMPILER="clang" \
+   -DCMAKE_CXX_COMPILER="clang++" \
+   -DCMAKE_C_FLAGS="-fprofile-instr-generate -fcoverage-mapping" \
+   -DCMAKE_CXX_FLAGS="-fprofile-instr-generate -fcoverage-mapping" \
+   -DBUILD_FOR_FUZZING=ON
+cmake --build build # Use "-j N" here for N parallel jobs.
+```
+
+Running fuzz tests with one or more targets
+
+```shell
+# For single target run with the target of choice
+LLVM_PROFILE_FILE="$(pwd)/build/raw_profile_data/txorphan.profraw" ./build/test/fuzz/test_runner.py ../qa-assets/fuzz_corpora txorphan
+# If running for multiple targets
+LLVM_PROFILE_FILE="$(pwd)/build/raw_profile_data/%m_%p.profraw" ./build/test/fuzz/test_runner.py ../qa-assets/fuzz_corpora
+# Merge profiles
+llvm-profdata merge build/raw_profile_data/*.profraw -o build/coverage.profdata
+```
+
+Generate report:
+
+```shell
+llvm-cov show \
+    --object=build/bin/fuzz \
+    -Xdemangler=llvm-cxxfilt \
+    --instr-profile=build/coverage.profdata \
+    --ignore-filename-regex="src/crc32c/|src/leveldb/|src/minisketch/|src/secp256k1/|src/test/" \
+    --format=html \
+    --show-instantiation-summary \
+    --show-line-counts-or-regions \
+    --show-expansions \
+    --output-dir=build/coverage_report \
+    --project-title="Bitcoin Core Fuzz Coverage Report"
+```
+
+The generated coverage report can be accessed at `build/coverage_report/index.html`.
+
 ### Performance profiling with perf
 
 Profiling is a good way to get a precise idea of where time is being spent in