Rename the fullTest cache key to fullBuild in CI, and document it

.github/workflows/main.yml

@@ -43,8 +43,13 @@ jobs:
         with:
           rust-version: stable${{ matrix.host }}
           targets: ${{ matrix.target }}
+      # The `{ sharedKey: ... }` allows different actions to share the cache.
+      # We're using a `fullBuild` key mostly as a "this needs to do the
+      # complete" that needs to do the complete build (that is, including
+      # `--features 'bundled-full session buildtime_bindgen`), which is very
+      # slow, and has several deps.
       - uses: Swatinem/rust-cache@v1
-        with: { sharedKey: fullTests }
+        with: { sharedKey: fullBuild }
 
       - run: cargo build --features bundled --workspace --all-targets --verbose
       - run: cargo test --features bundled --workspace --all-targets --verbose
@@ -83,7 +88,7 @@ jobs:
           rust-version: stable${{ matrix.host }}
           targets: ${{ matrix.target }}
       - uses: Swatinem/rust-cache@v1
-        with: { sharedKey: fullTests }
+        with: { sharedKey: fullBuild }
 
       - run: cargo test --features 'bundled-sqlcipher' --workspace --all-targets --verbose
       - run: cargo test --features 'bundled-sqlcipher' --workspace --doc --verbose
@@ -185,7 +190,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: hecrj/setup-rust-action@v1
       - uses: Swatinem/rust-cache@v1
-        with: { sharedKey: fullTests }
+        with: { sharedKey: fullBuild }
       - run: cargo doc --features 'bundled-full session buildtime_bindgen' --no-deps
         env: { RUSTDOCFLAGS: -Dwarnings }
 
@@ -195,6 +200,9 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - uses: hecrj/setup-rust-action@v1
+      # TODO: we don't use caching here because it's unclear if it will cause
+      # the coverage to get less accurate (this is the case for some coverage
+      # tools, although possibly not tarpaulin?)
       - name: Run cargo-tarpaulin
         uses: actions-rs/tarpaulin@v0.1
         with: