Update textbook.fr.md

00.brainstorming-pedagogical-teams/40.collection-existing-pedagogical-content/05.classical-mechanics/vector-analysis/textbook.fr.md

@@ -260,12 +260,14 @@ $`M=M(\alpha, \beta, \gamma)`$.
 [EN] There are *several possible ways to define spatial coordinates*: We speak of 
 ** coordinate systems**.
 
-* [ES]
+* [ES] si hay una ambigüedad con t, por ejemplo si t se usa para la densidad.
 [FR]
 [EN] Specific alphanumeric characters are defined for some widely used coordinate systems :<br>
 \- cartesian : $`(x, y, z)`$ or $`(O, x_1, x_2, x_3)`$<br>
-\- cylindric : $`(\rho, \phi, z)`$ or $`(r, \phi, z)`$<br>
-\- spheric : $`(x, y, z)`$ or $`(O, x_1, x_2, x_3)`$<br>
+\- cylindrical https://en.wikipedia.org/wiki/ISO_31-11 :<br>
+$`(\rho, \phi, z)`$ (or $`(r, \phi, z)`$ if there is an ambiguity with $`\rho`$, 
+for example if $`\rho`$ is used for density (electric) charge density).<br>
+\- spherical : $`(\rho, \phi, z)`$<br>
 
 
 #### Características de una base / Caractéristiques d’une base et d’un repère / Characteristics of a base