Improve lockfile concept documentation, add coverage for upgrades (#6698)

docs/concepts/projects.md

@@ -69,7 +69,7 @@ use [`uvx`](../guides/tools.md) or
     managed = false
     ```
 
-## Lockfile
+## Project lockfile
 
 uv creates a `uv.lock` file next to the `pyproject.toml`.
 
@@ -77,23 +77,6 @@ uv creates a `uv.lock` file next to the `pyproject.toml`.
 installed across all possible Python markers such as operating system, architecture, and Python
 version.
 
-If your project supports a more limited set of platforms or Python versions, you can constrain the
-set of solved platforms via the `environments` setting, which accepts a list of PEP 508 environment
-markers. For example, to constrain the lockfile to macOS and Linux, and exclude Windows:
-
-```toml title="pyproject.toml"
-[tool.uv]
-environments = [
-    "sys_platform == 'darwin'",
-    "sys_platform == 'linux'",
-]
-```
-
-Entries in the `environments` setting must be disjoint (i.e., they must not overlap). For example,
-`sys_platform == 'darwin'` and `sys_platform == 'linux'` are disjoint, but
-`sys_platform == 'darwin'` and `python_version >= '3.9'` are not, since both could be true at the
-same time.
-
 Unlike the `pyproject.toml`, which is used to specify the broad requirements of your project, the
 lockfile contains the exact resolved versions that are installed in the project environment. This
 file should be checked into version control, allowing for consistent and reproducible installations
@@ -110,10 +93,38 @@ The lockfile is created and updated during uv invocations that use the project e
 There is no Python standard for lockfiles at this time, so the format of this file is specific to uv
 and not usable by other tools.
 
+### Checking if the lockfile is up-to-date
+
 To avoid updating the lockfile during `uv sync` and `uv run` invocations, use the `--frozen` flag.
 
-To assert the lockfile is up-to-date, use the `--locked` flag. If the lockfile is not up-to-date, an
+To assert the lockfile matches the project metadata, use the `--locked` flag. If the lockfile is not
-error will be raised instead of updating the lockfile.
+up-to-date, an error will be raised instead of updating the lockfile.
+
+### Upgrading locked package versions
+
+By default, uv will prefer the locked versions of packages when running `uv sync` and `uv lock`.
+Package versions will only change if project's dependency constraints exclude the previous, locked
+version. To upgrade to the latest package versions supported by the project's dependency
+constraints, use `uv lock --upgrade`.
+
+### Limited resolution environments
+
+If your project supports a more limited set of platforms or Python versions, you can constrain the
+set of solved platforms via the `environments` setting, which accepts a list of PEP 508 environment
+markers. For example, to constrain the lockfile to macOS and Linux, and exclude Windows:
+
+```toml title="pyproject.toml"
+[tool.uv]
+environments = [
+    "sys_platform == 'darwin'",
+    "sys_platform == 'linux'",
+]
+```
+
+Entries in the `environments` setting must be disjoint (i.e., they must not overlap). For example,
+`sys_platform == 'darwin'` and `sys_platform == 'linux'` are disjoint, but
+`sys_platform == 'darwin'` and `python_version >= '3.9'` are not, since both could be true at the
+same time.
 
 ## Managing dependencies
 
@@ -151,6 +162,20 @@ To update an existing dependency, e.g., to add a lower bound to the `httpx` vers
 $ uv add 'httpx>0.1.0'
 ```
 
+!!! note
+
+    "Updating" a dependency refers to changing the constraints for the dependency in the
+    `pyproject.toml`. The locked version of the dependency will only change if necessary to
+    satisfy the new constraints. To force the package version to update to the latest within
+    the constraints, use `--upgrade-package <name>`, e.g.:
+
+    ```console
+    $ uv add 'httpx>0.1.0' --upgrade-package httpx
+    ```
+
+    See the [lockfile](#upgrading-locked-package-versions) section for more details on upgrading
+    package versions.
+
 Or, to change the bounds for `httpx`:
 
 ```console