Added database context propagation supported functions list to each database instrumentation library README (#450)

Author: jerrytfleung

src/Instrumentation/MySqli/README.md

@@ -59,4 +59,10 @@ OTEL_PHP_DISABLED_INSTRUMENTATIONS=mysqli
 Enable context propagation for database queries by installing the following packages:
 ```shell
 composer require open-telemetry/opentelemetry-sqlcommenter
-```
+```
+Currently, only the following functions support context propagation:
+* `mysqli::query`
+* `mysqli_query`
+* `mysqli::real_query`
+* `mysqli_real_query`
+

src/Instrumentation/PDO/README.md

@@ -37,7 +37,10 @@ OTEL_PHP_INSTRUMENTATION_PDO_DISTRIBUTE_STATEMENT_TO_LINKED_SPANS=true
 
 ## Database Context Propagation
 
-Enable context propagation for database queries by installing the following packages:
+Enable context propagation for database queries (pdo_mysql, pdo_pgsql only) by installing the following packages:
 ```shell
 composer require open-telemetry/opentelemetry-sqlcommenter
-```
+```
+Currently, only the following functions support context propagation:
+* `PDO::query`
+* `PDO::exec`

src/Instrumentation/PostgreSql/README.md

@@ -73,6 +73,9 @@ Enable context propagation for database queries by installing the following pack
 ```shell
 composer require open-telemetry/opentelemetry-sqlcommenter
 ```
+Currently, only the following functions support context propagation:
+* `pg_query`
+* `pg_send_query`
 
 ## Compatibility