Document that YAML files are expected to be in YAML1.2

Refer iterative/dvc#4415

content/docs/command-reference/metrics/index.md

@@ -69,10 +69,10 @@ stages:
 
 ### Supported file formats
 
-Metrics can be organized as tree hierarchies in JSON or YAML files. DVC
-addresses specific metrics by the tree path. In the JSON example below, five
-metrics are presented: `train.accuracy`, `train.loss`, `train.TN`, `train.FP`
-and `time_real`.
+Metrics can be organized as tree hierarchies in JSON or YAML files (specifically
+in YAML 1.2 format). DVC addresses specific metrics by the tree path. In the
+JSON example below, five metrics are presented: `train.accuracy`, `train.loss`,
+`train.TN`, `train.FP` and `time_real`.
 
 ```json
 {

content/docs/command-reference/params/index.md

@@ -32,6 +32,9 @@ organized as a tree hierarchy inside, as DVC will locate param names by their
 tree path. parameters files have to be manually written, or generated, and these
 can be versioned directly with Git.
 
+> Note that YAML files including `params.yaml` are expected to be in YAML 1.2
+> format.
+
 Supported parameter _value_ types are: string, integer, float, and arrays. DVC
 itself does not ascribe any specific meaning for these values. They are
 user-defined, and serve as a way to generalize and parametrize an machine

content/docs/command-reference/plots/index.md

@@ -48,9 +48,9 @@ differences between the metrics in different experiments.
 
 ### Supported file formats
 
-Plot metrics can be organized as data series in JSON, YAML, CSV, or TSV files.
-DVC expects to see an array (or multiple arrays) of objects (usually _float
-numbers_) in the file.
+Plot metrics can be organized as data series in JSON, YAML (in YAML 1.2 format),
+CSV, or TSV files. DVC expects to see an array (or multiple arrays) of objects
+(usually _float numbers_) in the file.
 
 In tabular file formats such as CSV and TSV, each column is an array.
 `dvc plots` subcommands can produce plots for a specified column or a set of

content/docs/user-guide/dvc-files-and-directories.md

@@ -16,9 +16,9 @@ Additionally, there are a few metafiles that support DVC's features:
   These typically have a matching `dvc.lock` file to record the pipeline state
   and track its <abbr>data artifacts</abbr>.
 
-Both `.dvc` files and `dvc.yaml` use human-friendly YAML schemas, described
-below. We encourage you to get familiar with them so you may create, generate,
-and edit them on your own.
+Both `.dvc` files and `dvc.yaml` use human-friendly YAML schemas (expected to be
+in YAML 1.2 format), described below. We encourage you to get familiar with them
+so you may create, generate, and edit them on your own.
 
 Both the internal directory and these metafiles should be versioned with Git (in
 Git-enabled <abbr>repositories</abbr>).